fix(github): add labels and milestone to PR checklist

[edward-ly]

• Follow-up to fix(comments): expand long comment on click #55179 (comment)

Summary

Creates reminders to add labels and milestones to PRs (see last two items in checklist below).

For neuro-accessibility reasons, required items should be explicitly rather than implicitly said.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

[edward-ly]

/backport to stable32

[edward-ly]

/backport to stable31

[AndyScherzinger]

Thanks for taking the time and making the effort 👍

[backportbot]

The backport to stable31 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable31
git pull origin stable31

# Create the new backport branch
git checkout -b backport/55237/stable31

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick b77f085d

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/55237/stable31

Error: Failed to push branch backport/55237/stable31: remote: {"auth_status":"auth_error","body":"Invalid username or token. Password authentication is not supported for Git operations."}
fatal: Authentication failed for 'https://github.com/nextcloud/server.git/'

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.