CI: Prevent breaking backports (#4812)
- Prevent `major` changes to be merged into a `stable` branch.
- Place a comment on backport MRs to provide context of what it means.
Comment looks like this:
![Screenshot 2024-07-24 at 17 36
35](https://github.com/user-attachments/assets/6393549b-7b15-41e5-a804-8581c625ceff)
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Signed-off-by: Oliver Tale-Yazdi <[email protected]>