Release 0.9 beta

[KitsuneRal]

Pre-check:

• [x] There are no pending issues in the Roadmap for this release (if there are, move them away or close them)
• [x] The repo is on the correct branch/commit
• [x] CI:
• [x] Rebuild (from scratch) all targets with GCC and check autotests+quotest

Actions:

• [x] Make a branch for the release, and check/update:
  • [x] the version number in CMakeLists.txt
• [x] Go to the Releases page and make a draft for release notes:
  • [x] Fill in the tag (should be "N.M.L", NOT "N.M")
  • [x] Thank all contributors whose commits landed in this release
  • [x] Describe all notable fixes/enhancements/API breakage
• [x] git commit && git push, if not yet
  • [x] After an hour, check CI for the release branch; if it failed, investigate and decide on a further action
  • [x] Open a PR, with "Closes" on the issue with the checklist
  • [x] After another hour :-\ check CI on the PR
  • [x] Merge the branch
• [x] Publish release notes at GitHub (the title should be "###", not "Version ###")
• [x] Update the topic in #quotient
• [x] Post a TWIM update

Post-check:

• [x] Open the release page in Incognito/Privacy mode; check the version and the Git tag