Use towncrier for changelog generation

[MetRonnie]

https://towncrier.readthedocs.io/en/stable/index.html

If we go for this it means no more changelog conflicts 🎉 Instead you either

• write your changelog entries to a file at changes.d/<PR-number>.<break|feat|fix>.md
• or run towncrier create <PR-number>.<break|feat|fix>.md --content "<insert content here>"

and before the release they get collated into CHANGES.md.

Also it guarantees changelog entries go under the right release section despite any milestone bump-backs or cherry-picks that happen.

Closes #4356

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).
• Dependency changes do not apply to conda-environment.yml
• Tests N/A
• No changelog entry needed
• No cylc-doc change needed

[MetRonnie]

Here is the towncrier build generated changelog section from a bunch of example fragment files I added. (Normally the fragment files get deleted when you run towncrier build [--yes])

[MetRonnie]

In response to a question Dave asked: although these changelog fragments will be included into the master branch in sync PRs, once we do a maintenance release and subsequent sync PR, the deleted fragments will also be removed on master so you won't get duplicate entries under maintenance and minor sections in CHANGES.md

[oliver-sanders]

Willing to give it a go.

[hjoliver]

Nice 👍