workflow-state: query latest task instance

[hjoliver]

When a task gets updated in the task states table, update the flow numbers too.

Close #5143

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 both setup.cfg and conda-environment.yml.
• [ ] Tests are included (or explain why tests are not needed).
• [ ] CHANGES.md entry included if this is a change that can affect users
• [ ] Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• [ ] If this is a bug fix, PRs raised to both master and the relevant maintenance branch.

[hjoliver]

Rebased to 8.0.x

[oliver-sanders]

Rebased to 8.0.x

@hjoliver, can you poke this onto 8.2.x (I think 8.2.x is currently master)

[hjoliver]

Closing this as superseded by #5809