x/tools/gopls: release version v0.7.5

[findleyr]

This issue tracks progress on releasing x/tools/gopls@v0.7.5.

• reset the x/tools replace directive
• merge master into gopls-release-branch.0.7
• update hard-coded version
• update go.mod/go.sum (remove x/tools replace, update x/tools version)
• tag prerelease gopls/v0.7.5-pre.1
• update Github milestone
• write release notes
• smoke test features
• tag gopls/v0.7.5

CC @hyangah @pjweinbgo @suzmue

[gopherbot]

Change https://golang.org/cl/378582 mentions this issue: gopls: v0.7.5: add replace directive back to go.mod

[gopherbot]

Change https://golang.org/cl/378585 mentions this issue: internal/lsp/debug: update hard-coded version for v0.7.5

[gopherbot]

Change https://golang.org/cl/379115 mentions this issue: gopls: update go.mod file for v0.7.5

[findleyr]

Release is complete: https://github.com/golang/tools/releases/tag/gopls%2Fv0.7.5