Show relevant changes in the platform to the user.

[xrstf]

What would you like to be added? / User Story:
For some releases, we had a changelog functionality in KKP. This was done by keeping a subset of the changes for a release in form of a changelog.json in this repository. This had 2 major downsides:

1. We had to make a guess as to which changes from the entire changelog to include in the "user-visible" changelog.json in this repository. Whoever curated the changelog for a new release had to make a choice what to include.
2. Since we only ever kept the changes for that patch version (i.e. KKP 2.25.4's changelog.json only contained the changes for that .4 patch release), users would basically never see all relevant changes. If an admin upgrades KKP from 2.25.5 to 2.26.1, end-users would see only the .1 changes and all the big new features and improvements for 2.26.0 would be missed.

For these reasons we removed the entire functionality in #6631. However in general the idea of informing end-users about relevant changes is still good and with a bit of refinement, we should bring it back.

[xrstf]

While thinking about this, we could also think about a way of including a notification mechanism for the KKP platform owner. So they could broadcast a message like "Seed X is dead for the weekend" or "Ubuntu no worky on AWS today." and have it shown as a banner on top.