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)

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>