x/tools/gopls: formally deprecate experimentalWorkspaceModule

[findleyr]

With go.work files, we no longer need the experimentalWorkspaceModule feature for gopls, which like its name suggests never got out of experimental (it was superseded by workspaces).

We should start alerting users of experimentalWorkspaceModule to switch to go.work, and plot a roadmap to full removal of this feature. As is, our users can encounter bugs in experimentalWorkspaceModule that don't exist with go.work files (e.g. golang/vscode-go#2243).

[gopherbot]

Change https://go.dev/cl/434640 mentions this issue: gopls: deprecate three experimental features