Update MacOS version on GH Actions

[MetRonnie]

Includes DNS patch

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] Infrastructure changes so no need for any funny stuff
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[MetRonnie]

Lint failure fixed in #6188

[oliver-sanders]

Some nasty looking traceback from two tests (which both passed):

• https://github.com/cylc/cylc-flow/actions/runs/9808161608/job/27083401819?pr=6187#step:16:118
• https://github.com/cylc/cylc-flow/actions/runs/9808161608/job/27083402044?pr=6187#step:16:46

Just need to quickly check that the tests aren't giving a false negative, then we can follow up in a new issue.

~(Kicked off CI run (1/5, 2/5) configured to fail all jobs so we can see the logs which should reveal where that traceback is coming from)~

[oliver-sanders]

Opened separate issue for tracebacks: https://github.com/cylc/cylc-flow/issues/6208