cylc clean: rename --force to --yes

[MetRonnie]

These changes partially address #4673 and #4618

The --force behaviour is more of a --yes than --force, so rename it.

--force can come back later if we want to allow users to override the current "cannot clean" scenarios - see #4450

Requirements 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.
• Appropriate tests: Add func test for multi clean #4682
• Appropriate change log entry included.
• Docs PR: Update cylc clean docs cylc-doc#403

[oliver-sanders]

partially address #4673 and #4618

What's left?

[MetRonnie]

The second task on #4673

As for #4618, I've changed the multi workflow clean message

-Would remove the following workflows:
+Would clean the following workflows:

which is better because it doesn't explicitly say the whole run dir will be removed when you're only doing --rm subdir, but it still kinda implies it, so have left the issue open

[oliver-sanders]

So closes #4618?

[MetRonnie]

I still think the prompt message is a bit alarming for targeted clean, perhaps the prompt could do with a slight rethink later on

[oliver-sanders]

I think it's ok, to explain better we would need something like a dry run listing exactly what we would expect to be removed where (perfectly possible though probs a bit OTT).

[oliver-sanders]

Very small scope, can single review this.