Golang x/tools/gopls: release version v0.20.0

This issue tracks progress toward releasing gopls@v0.20.0

• [ ] create or update gopls-release-branch.0.20
• [ ] update go.mod/go.sum (remove x/tools replace, update x/tools version)
• [ ] tag gopls/v0.20.0-pre.1
• [ ] update GitHub milestone
• [ ] write release notes
• [ ] smoke test features
• [ ] tag gopls/v0.20.0
• [ ] (if vX.Y.0 release): update dependencies in master for the next release