x/tools/gopls: release version v0.18.1

[gopherbot]

This issue tracks progress toward releasing gopls@v0.18.1

• create or update gopls-release-branch.0.18
• update go.mod/go.sum (remove x/tools replace, update x/tools version)
• tag gopls/v0.18.1-pre.1
• update Github milestone
• write release notes
• smoke test features
• tag gopls/v0.18.1
• (if vX.Y.0 release): update dependencies in master for the next release

[gopherbot]

Change https://go.dev/cl/651217 mentions this issue: gopls: update go.mod for v0.18.1-pre.1

[gopherbot]

Change https://go.dev/cl/651218 mentions this issue: extension: update gopls v0.18.1-pre.1 settings

[gopherbot]

Change https://go.dev/cl/652115 mentions this issue: gopls: update go.mod for v0.18.1-pre.2

[gopherbot]

Change https://go.dev/cl/652077 mentions this issue: extension: update gopls v0.18.1 settings