(TEST) Prepare release: 8.9.10

[github-actions]

⚡ Merging this PR will automatically create a GitHub Release & publish the package ⚡

This PR was created by the Release stage 1 - create release PR (Cylc 8+ only) workflow, triggered by @MetRonnie

Tests:

• Tests last run on master:
• ✔️ Build check passed - see the workflow run (number 10) for more info

Checklist:

• Milestone complete?
  ⚠️ Couldn't find milestone matching 8.9.10
• Changelog up-to-date?
  • Examine pull requests made since the last release
  • "Released on" date updated? ⚠️ couldn't automatically set date

Next steps:

• @MetRonnie should request 1 or 2 reviews
• If any further changes are needed, push to this PR branch
• After merging, the bot will comment below with a link to the release (if not, look at the PR checks tab)