x/tools/gopls: match new language versions for type checking

[findleyr]

Just spotted this bug: https://cs.opensource.google/go/x/tools/+/master:gopls/internal/lsp/cache/check.go;l=1490;drc=6de34480a867b1198bb6c910cb77c147e89a6023

We need to update goVersionRx to allow for e.g. go1.21.1. As is, it looks like we'll be setting the wrong language version for the new version strings.

[findleyr]

This is harmless until 1.22 comes out, because the default behavior of the type checker is to allow all features.

[gopherbot]

Change https://go.dev/cl/557215 mentions this issue: gopls/internal/lsp/cache: allow versions of form "go1.2.3"