chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/pmtud/tcp/multi.go

@@ -12,11 +12,6 @@ import (
 	"github.com/qdm12/gluetun/internal/pmtud/test"
 )
 
-var (
-	ErrMTUNotFound = errors.New("MTU not found")
-	ErrMSSTooSmall = errors.New("TCP MSS is too small to find the MTU")
-)
-
 type testUnit struct {
 	mtu uint32
 	ok  bool
@@ -178,5 +173,5 @@ func pathMTUDiscover(ctx context.Context, fd fileDescriptor,
 		}
 	}
 
-	return 0, fmt.Errorf("%w: your connection might not be working at all", ErrMTUNotFound)
+	return 0, errors.New("MTU not found: your connection might not be working at all")
 }