Changes and release notes g/g/ext -> extensions

[timebertt]

What would you like to be added:

The gardener extensions library (https://github.com/gardener/gardener/tree/master/extensions) contains code commonly used across all our extensions by vendoring g/g.

We would like to have

• a dedicated release note type for changes in g/g/ext that affect extension developers and that contain improvements for all extension controllers (e.g. in the generic worker actuator) -> release note category dependency was added
• upgrade PRs to the extension repositories vendoring newer versions of g/g: Automated g/g upgrade PRs in extension repositories after a new release of g/g gardener#6328
  • automatically triggered for releases of g/g
  • manually triggered for master branch of g/g
• automated transport of g/g/ext release notes with these upgrade PRs

Why is this needed:
Doing the mentioned maintenance work for all our extensions manually is tedious and also a lot of effort, that can be automated (at least to a very high portion).
Also properly marking and transporting release notes is necessary to improve transparency for extension developers and end users.

[timebertt]

/cc @gardener/gardener-maintainers