x/tools/gopls: release version v0.12.0

[findleyr]

This issue tracks progress toward releasing gopls@v0.12.0. (Note that this release constitutes a major internal change to gopls, and so this issue will remain open for a while, unlike other release tracking issues).

• create gopls-release-branch.0.12
• merge master into gopls-release-branch.0.12
• update hard-coded version
• update go.mod/go.sum (remove x/tools replace, update x/tools version)
• tag gopls/v0.12.0-pre.1
• tag gopls/v0.12.0-pre.2
• tag gopls/v0.12.0-pre.3
• update Github milestone
• write release notes
• smoke test features
• tag gopls/v0.12.0
• update dependencies in master for the next release

[gopherbot]

Change https://go.dev/cl/488335 mentions this issue: codereview.cfg: update for gopls-release-branch.0.12

[gopherbot]

Change https://go.dev/cl/488355 mentions this issue: gopls/internal/lsp/debug: update hard-coded version for v0.12.0

[gopherbot]

Change https://go.dev/cl/488415 mentions this issue: gopls: update go.mod for v0.12.0

[gopherbot]

Change https://go.dev/cl/495260 mentions this issue: all: merge master (743372f) into gopls-release-branch.0.12

[gopherbot]

Change https://go.dev/cl/495262 mentions this issue: gopls: update go.mod for v0.12.0-pre.2

[gopherbot]

Change https://go.dev/cl/497957 mentions this issue: all: merge master (9c97539) into gopls-release-branch.0.12