x/pkgsite, x/build/cmd/relui: automate fetching of new versions of Go standard library

[dmitshur]

This issue is related to #37002, but limited in scope to just the Go standard library.

When a new version of Go is released, it takes a manual action to add it to pkg.go.dev. This can and should be automated, so that it happens as soon as a new Go version is released.

/cc @julieqiu @jba @cagedmantis @toothrot @FiloSottile

[dmitshur]

One of the ways this can be implemented is by using maintner to watch for when new Go release tags are pushed. I opened #37577 about potentially documenting that process so it's more discoverable.