We should surface a deprecation notice for Go 1.16 and Go 1.17 in gopls@v0.12.0, which is going out soon.

Per our documented support policy, we nominally support using gopls with the last 4 Go releases (notably longer than the 2 releases supported by Go itself). Note that we want to eventually narrow that support window (#50825), but will probably wait for the forward compatibility proposal before doing so.

Our next major release (v0.13.0) is likely to be at or around the Go 1.21 release, at which point the four previous major versions will be Go 1.18-21. Furthermore, survey results indicate only a tiny fraction of users are still on 1.17. Therefore, I think it is prudent to surface this deprecation notice now, and remove support in gopls@v0.13.0.

This also means that once we cut the v0.12.0 release in gopls, we can start using all the great new features of Go 1.18!

CC @adonovan