fix broadcast cancel error

[oliver-sanders]

• Closes cylc broadcast --cancel TypeError when cancelling [events] setting #6950
• Disable an upgrader-patched validation check which is incompatible with cylc broadcast --cancel.

So cylc broadcast runs config fragments via cylc.flow.cfgspec.workflow in order to run them through the upgraders.

Cute, means you can still broadcast deprecated settings, not sure why we support that though?

We have patched some validation stuff into the upgrader (naughty) which is incompatible with this usage.

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 if present).
• Tests are included (or explain why tests are not needed).
• Changelog 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, PR should be raised against the relevant ?.?.x branch.

[MetRonnie]

I've pushed a changelog entry

[matthewrmshin]

Thanks for fixing.