x/tools/gopls: release version v0.15.2

[findleyr]

This issue tracks progress toward releasing gopls@v0.15.2

• create or update gopls-release-branch.0.15
• update go.mod/go.sum (remove x/tools replace, update x/tools version)
• tag gopls/v0.15.2-pre.1
• update Github milestone
• write release notes
• smoke test features
• tag gopls/v0.15.2

[gopherbot]

Change https://go.dev/cl/569881 mentions this issue: gopls: update go.mod for v0.15.2-pre.1

[gopherbot]

Change https://go.dev/cl/570756 mentions this issue: gopls: update go.mod for v0.15.2-pre.2