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