Fix lack of errors/warnings for deprecated [runtime][<task>][remote]retrieve job logs * settings

[MetRonnie]

These changes close #4944.

• Setting [runtime][<task>][remote]retrieve job logs = false on a task specified in the old way results in a "No platform found matching your task" error (my platform definition for the matching host has log retrieval enabled) . This appears to working as intended.
• Setting [runtime][<task>][remote]retrieve job logs = false on a task specified using a platform does not result in a validation error so this needs fixing.

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.
• Updated tests
• Change log entry
• No cylc-doc PR required.

[MetRonnie]

Why isn't there a default value for this (True or False)?

[oliver-sanders]

Because history I'm afraid. We should make these defaults explicit - #4847

[hjoliver]

(Review pending response to this: #4944 (comment) )

[hjoliver]

From @dpmatthews explanatory comment on #4944:

Setting [runtime][<task>][remote]retrieve job logs = false on a task specified using a platform does not result in a validation error so this needs fixing.

So I guess you can just fix that here @MetRonnie, and we're good to go.

[hjoliver]

(Some functional test failures)

[MetRonnie]

cylc-flow/tests/functional/events/11-cycle-task-event-job-logs-retrieve.t

Lines 31 to 32 in 19bdbf5

	# There are 2 remote tasks. One with "retrieve job logs = True", one without.
	# Only t1 should have job.err and job.out retrieved.

Not sure how to achieve this, I think a solution may be:

• Do not deprecate flow.cylc[runtime][task][remote]retrieve job logs in favour of platforms
• Instead, move it up a section
• This will override the platform default
• Otherwise, the retrieve job logs max size and retrieve job logs retry delays should be deprecated in favour of platforms

After discussing, solution is to check blame and either stop using platforms for this test or remove the test

[wxtim]

• Looks sane
• Local testing suggests that it works

[hjoliver]

👍