Automate release process

[tklauser]

The current release process as documented in RELEASE.md still contains quite some manual steps to be performed by a maintainer. Try to automate these as much as possible.

Quoting @nbusseneau from #434 (review):

Thoughts: I'm thinking we could automate almost everything in there. Full process could be:

• A workflow that needs to be manually triggered, passing in version parameters. When triggered:

  • Prepare release notes.
  • Update stable.txt and VERSION.
  • Push branch and create PR.

• Then, another workflow running on push to master:

  • Identify that a release PR just got merged (file names filtering? commit names? PR name?)
  • Tag using version from PR
  • Create GH release using release notes from PR

[tklauser]

@twpayne suggested to use goreleaser to automate the release process: #509 (comment)

[tklauser]

With the release process simplification in #469, #533 and #811 as well as the GitHub auto-generated release notes (#686), the release process now consists of only two manual steps: creating/pushing the tag, followed by checking/publishing the auto-generated release notes. We intentionally want to keep that final step manual, so closing this issue.