x/tools/gopls: publish v0.21.0 release notes, automate

[findleyr]

gopls@v0.21.0 is out, but there are no release notes:
https://github.com/golang/tools/releases

We should publish release notes (or advertise the release in some other way, if we don't want to use github releases anymore).

Now that our release notes are in the tree, perhaps we can automate a release publication in relui.

CC @adonovan

[gabyhelp]

Related Issues

• x/tools/gopls: release version v0.21.0 #76367 (closed)
• x/tools: publish GitHub releases for tools #64886 (closed)
• x/tools, x/build: create a relui workflow for gopls release automation #57643 (closed)
• x/tools/gopls: release version v0.20.0 #74654 (closed)
• x/vuln/cmd/govulncheck: publish release notes #65945 (closed)

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