Add "Backwards compatibility" notes to release notes

[BigRoy]

Is your feature request related to a problem? Please describe.

I mentioned in this PR that the settings change likely was a backwards incompatible change. As far as I can see any settings a studio had defined there previously will likely not work for the newer version and thus their values will be removed.

Currently that PR made its way into that 3.15.3 version but looking at the release notes it's pretty unclear that that feature might introduce some incompatibilities for existing settings.

Describe the solution you'd like

I would expect as an admin that I could take a glimpse over new releases and be able to quickly see "Update notes" or "Backwards compatibily" warnings so that I know as a studio or admin where to pay attention too.

Should we maybe have a tag on PRs like backwards incompatibility* - maybe one labeled in general and maybe one labeled for settings.

Then PRs could have a default block for ### Backwards compatibility or alike and any info in there could make its way into a clearly labeled area in release notes like these: https://github.com/ynput/OpenPype/releases/tag/3.15.3

Preferably in such a way that an admin or someone updating could just quickly see what he/she might need to keep in mind when updating?

Describe alternatives you've considered

Never be backwards incompatible or 🙉 🙈 🙊 . Or where being backwards incompatible implement automatic conversions so that it becomes forward compatible.

[cuID:OP-5430]