x/tools/gopls: drop support for building at Go 1.12

[findleyr]

As discovered in CL 380555, some of our dependencies no longer support building with Go 1.12. This is to be expected -- 1.12 was a long time ago.

#50825 outlines a long-term plan to remove support for building gopls at legacy Go versions, but in the meantime we need to to mark gopls@v0.7.5 as the final release supporting being built by Go 1.12. Fortunately, this is consistent with our stated support for the four latest major Go versions.

Action items:

• check with the VS Code team to see if there's anything we can do to help users avoid breakage when upgrading gopls from VS Code (CC @hyangah @suzmue)
• document our new compatibility matrix.
• update the v0.7.5 release notes
• remove Kokoro 1.12 CI
• upgrade our dependencies, thus breaking the 1.12 build

[gopherbot]

Change https://golang.org/cl/382075 mentions this issue: gopls: document that v0.7.5 is the final version to support Go 1.12

[gopherbot]

Change https://golang.org/cl/380555 mentions this issue: gopls: update dependencies post-release

[gopherbot]

Change https://go.dev/cl/401155 mentions this issue: internal/jsonrpc2*: remove usage of golang.org/x/xerrors

[gopherbot]

Change https://go.dev/cl/401097 mentions this issue: gopls: remove usage of golang.org/x/xerrors

[gopherbot]

Change https://go.dev/cl/401154 mentions this issue: go/packages: remove usage of golang.org/x/xerrors