demote unnecessary warnings

[wxtim]

Closes #6760

https://github.com/cylc/cylc-ui/pull/2169 adds streaming warnings to the UI, but this highlights some warnings which may be either very large, or of no interest to the user.

I am aware that this change may engender controversy - feel free to persuade me that my choices are wrong.

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• [x] Refactor and log level change - should be covered by existing tests, and indeed a number broke.
• [x] ~Changelog entry included if this is a change that can affect users~ No: These changes are being made because these warning should NOT affect users.
• [x] ~Cylc-Doc~ N/A
• [x] Not a bug fix => master

[wxtim]

I think that I'm up-to-date with this one - there are still some half-hearted discussions - can I ask @hjoliver and @oliver-sanders to review comments?

[oliver-sanders]

FYI: Have checked back through the comments, I think we're all agreed on these changes.