Bump minimum supported Python version to 3.12

[MetRonnie]

Sibling: cylc/cylc-flow#6809

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to setup.cfg (no conda-environment.yml present).
• Tests are not needed
• Changelog entry included if this is a change that can affect users
• Minimum Python 3.12 cylc-doc#866
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.