PR Template Updates

# 🐜 Bug Report

The PR template needs to be updated to accommodate changing from Trac to GitHub for core ANTS. This still needs to account for contrib being on Trac for now, so there will need to be another iteration once we've migrated contrib too.

1. We should have a summary at the top for what the PR is intended to do (which may be just "see the issue"), and for any additional comments. We currently have this about 2/3 of the way down the page, which made sense on Trac when the ticket description was front and centre. But this should be more prominent on GitHub.
2. The `code branches` line in Branch section is obsolete.
3. The rose stem logs in the Branch section need to be updated: it needs to include the option that the rose stem workflows were not run (external collaborators will typically be unable to run the suites). I think there's an argument that these rose stem log lines are simply obsolete and the statuses at the bottom of the template are sufficient?
4. In the approvals section, can we amend Ancil Science Owner to be `NAME/NA` rather than just `NAME`
5. In general, does GitHub markdown give us a better way to do `YES/NO/NA`?
6. In the `Other` section, the Contributor Licence Agreement section should be updated. It now applies to Met Office people too, and isn't signed and returned.
7. All links to the CLA should be updated. There's at least 2 links.
8. Can we lose the angle brackets in ``?
9. For the contrib trac status log, can we advise people to remove user names and paths?
10. For the contrib trac status log, can we provide instructions for translating from trac wiki to Markdown? This may be via e.g. `sed` commands, or via rerunning the trac status generator to generate markdown (note this would involve update the status generator - we still need the trac wiki version for contrib tickets on trac though. Probably best to spin this off to a sub issue if it's the approach taken).
11. The line at the top with the ticket number? Can we include one of the closing keywords on it (e.g. `Ticket summary: Closes #NNNN` instead of `Ticket summary: #NNNN`)? Bonus points for changing from `Ticket` to `Issue`...