Remove mentions of Ubuntu 18.04

Even Ubuntu 20.04 is so old at this point that there's no real reason to
keep supporting 18.04.

.github/workflows/build.yml

@@ -18,54 +18,6 @@ defaults:
     shell: bash
 
 jobs:
-  build-bionic:
-    name: Build on Ubuntu 18.04
-    runs-on: ubuntu-latest
-    # This container contains everything needed to build yabridge
-    container: robbert/docker-yabridge:bionic
-    outputs:
-      artifact-name: ${{ env.ARCHIVE_NAME }}
-    # GitHub actions does not allow you to share steps between jobs and their
-    # yaml parser does not support anchors, so we'll have to duplicate all of
-    # these steps
-    # https://github.community/t5/GitHub-Actions/reusing-sharing-inheriting-steps-between-jobs-declarations/td-p/37849
-    steps:
-      - uses: actions/checkout@v3
-      # Needed for git-describe to do anything useful, the safe directory
-      # workaround is needed for https://github.com/actions/runner/issues/2033
-      - name: Fetch all git history
-        run: |
-          git config --global --add safe.directory /__w/yabridge/yabridge
-          git fetch --force --prune --tags --unshallow
-      - name: Determine build archive name
-        run: |
-          echo "ARCHIVE_NAME=yabridge-$(git describe --always)-ubuntu-18.04.tar.gz" >> "$GITHUB_ENV"
-      - name: Build the binaries
-        run: |
-          # Wine won't create a Wine prefix in ~/.wine because /github/home is
-          # not owned by the user that's executing this job
-          mkdir -p /tmp/prefix
-          export WINEPREFIX=/tmp/prefix
-
-          # Statically link to libstdc++ on Ubuntu 18.04 as we're compiling
-          # with a newer version of g++ than what's in the default repos
-          meson setup build --buildtype=release --cross-file=cross-wine.conf --unity=on --unity-size=10000 -Dbitbridge=true -Dcpp_link_args='-mwindows -static-libstdc++' -Dbuild.cpp_link_args='-static-libstdc++'
-          ninja -C build
-      - name: Strip remaining debug symbols
-        run: strip build/libyabridge{,-chainloader}-{clap,vst2,vst3}.so build/yabridge-host{,-32}.exe.so
-      - name: Create an archive for the binaries
-        run: |
-          mkdir yabridge
-          cp build/libyabridge{,-chainloader}-{clap,vst2,vst3}.so build/yabridge-host{,-32}.exe{,.so} yabridge
-          cp CHANGELOG.md README.md yabridge
-
-          tar -caf "$ARCHIVE_NAME" yabridge
-          rm -rf yabridge
-      - uses: actions/upload-artifact@v3
-        with:
-          name: ${{ env.ARCHIVE_NAME }}
-          path: ${{ env.ARCHIVE_NAME }}
-
   build-focal:
     name: Build on Ubuntu 20.04
     runs-on: ubuntu-latest
@@ -108,9 +60,11 @@ jobs:
           name: ${{ env.ARCHIVE_NAME }}
           path: ${{ env.ARCHIVE_NAME }}
 
-  # The same as `build-bionic`, but for a 32-bit only version of yabridge. We
-  # don't plan on distributing this, but we'll just have this available for AV
-  # Linux. We currently don't do 32-bit builds of yabridgectl though.
+  # This is still based on the now removed `build-bionic`, but for a 32-bit only
+  # version of yabridge. We don't plan on distributing this, but we'll just have
+  # this available for AV Linux. We currently don't do 32-bit builds of
+  # yabridgectl though, but as long as this keeps building there's no harm in
+  # keeping this around.
   build-32-bit:
     name: Build on Ubuntu 18.04 (32-bit build)
     runs-on: ubuntu-latest

README.md

@@ -94,10 +94,9 @@ Please let me know if there are any issues with other hosts.
      distro, you can simply download and install a prebuilt version of yabridge:
 
      1. First download the latest version of yabridge from the [releases
-        page](https://github.com/robbert-vdh/yabridge/releases). If you're using
-        a distro that's older than Ubuntu 20.04 such as Ubuntu 18.04, Debian 10,
-        Linux Mint 19, or AV Linux MXE 2021, then you should download the
-        version that ends with `-ubuntu-18.04.tar.gz`.
+        page](https://github.com/robbert-vdh/yabridge/releases). These binaries
+        currently target Ubuntu 20.04, and should work on any other distro
+        that's newer than that.
      2. Extract the contents of the downloaded archive to `~/.local/share`, such
         that the file `~/.local/share/yabridge/yabridgectl` exists after
         extracting. You can extract an archive here from the command line with
@@ -836,7 +835,7 @@ up the new changes._
 To compile yabridge, you'll need [Meson](https://mesonbuild.com/index.html) and
 the following dependencies:
 
-- GCC 10+[\*](#building-ubuntu-18.04)
+- GCC 10+
 - A Wine installation with `winegcc` and the development headers. The latest
   commits contain a workaround for a winelib [compilation
   issue](https://bugs.winehq.org/show_bug.cgi?id=49138) with Wine 5.7+.
@@ -867,17 +866,6 @@ ninja -C build
 After you've finished building you can follow the instructions under the
 [usage](#usage) section on how to set up yabridge.
 
-<sup id="building-ubuntu-18.04">
-  *The version of GCC that ships with Ubuntu 18.04 by default is too old to
-  compile yabridge. If you do wish to build yabridge from scratch rather than
-  using the <a
-  href="https://github.com/robbert-vdh/yabridge/actions?query=workflow%3A%22Automated+builds%22+branch%3Amaster">prebuilt
-  binaries</a>, then you should take a look at the <a
-  href="https://github.com/robbert-vdh/docker-yabridge/blob/master/bionic/Dockerfile">docker
-  image</a> used when building yabridge on Ubuntu 18.04 for on overview of what
-  would need to be installed to compile on Ubuntu 18.04.
-</sup>
-
 ### 32-bit bitbridge
 
 It is also possible to compile a host application for yabridge that's compatible