Upgrade towncrier

[MetRonnie]

Supersedes #6216

towncrier build will now fail as part of CI if you create a file in changes.d/ with an invalid fragment file name, thanks to my PR twisted/towncrier#622 (needs the ignore setting added to the towncrier config).

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).
• Dev dependency change does not apply to conda
• Tests are not needed
• No changelog entry needed
• No docs needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[MetRonnie]

(No need to run the functional tests so I've cancelled them)

[MetRonnie]

Test failures are unrelated, fixed by #6257