x/tools/gopls: release version v0.15.0

[findleyr]

This issue tracks progress toward releasing gopls@v0.15.0.

This release includes zero-config gopls (#57979) and improved handling of build tags (#29202). Since these are fundamental changes to gopls' internal data model, we will let this prerelease soak for some time, and will have multiple pre-releases.

• create or update gopls-release-branch.0.15
• update go.mod/go.sum (remove x/tools replace, update x/tools version)
• tag gopls/v0.15.0-pre.1
• tag gopls/v0.15.0-pre.2
• tag gopls/v0.15.0-pre.3
• tag gopls/v0.15.0-pre.4
• tag gopls/v0.15.0-pre.5 (the final prerelease)
• update Github milestone
• write release notes
• smoke test features
• tag gopls/v0.15.0
• (if vX.Y.0 release): update dependencies in master for the next release

[gopherbot]

Change https://go.dev/cl/554318 mentions this issue: all: update codereview.cfg for gopls-release-branch.0.15

[gopherbot]

Change https://go.dev/cl/554319 mentions this issue: gopls: update go.mod for v0.15.0-pre.1

[gopherbot]

Change https://go.dev/cl/561976 mentions this issue: all: merge master (a297bfd) into gopls-release-branch.0.15

[gopherbot]

Change https://go.dev/cl/562038 mentions this issue: gopls: update go.mod for v0.15.0-pre.2

[gopherbot]

Change https://go.dev/cl/563715 mentions this issue: all: merge master (1b39a8b) into gopls-release-branch.0.15

[gopherbot]

Change https://go.dev/cl/563775 mentions this issue: gopls: update go.mod for v0.15.0-pre.3