x/tools/gopls: release version v0.8.1

[findleyr]

gopls@v0.8.1 is a small patch release containing several fixes for Go 1.18 features.

• merge master into gopls-release-branch.0.8
• update hard-coded version
• update go.mod/go.sum (remove x/tools replace, update x/tools version)
• tag gopls/v0.8.1-pre.1
• update Github milestone
• write release notes
• smoke test features
• tag gopls/v0.8.1

[gopherbot]

Change https://go.dev/cl/392434 mentions this issue: all: merge master (49d48a0) into gopls-release-branch.0.8

[gopherbot]

Change https://go.dev/cl/392474 mentions this issue: internal/lsp/debug: update hard-coded version for v0.8.1

[gopherbot]

Change https://go.dev/cl/396474 mentions this issue: internal/lsp/debug: update hard-coded version for v0.8.2