community/go: upgrade to 1.21

Upstream only tests their musl stuff on x86_64 and we occasionally discover issues on other musl arches. Let's be a good citizen for once and test the release candidate on our CI arches so that we can report potential breakage early.

Also: From the preliminary release notes:

To make these new stricter version requirements easier to manage, the go command can now invoke not just the toolchain bundled in its own release but also other Go toolchain versions found in the PATH or downloaded on demand.

I am not sure how this works (e.g. is the downloaded Go binary linked against glibc) and we might want to patch that out I suppose?

To-Do:

• Patch out the "download other version of the compiler" feature?
• Package a /usr/lib/go/go.env
• Need to check if this still bootstraps with gccgo
  • https://github.com/golang/gofrontend/commit/68a756b6aadc901534cfddddad2b1e73fae9e34f
  • https://github.com/golang/go/issues/60913