Improve/fix release note generation

[Diaphteiros]

Description

Our PR template specifies a 'Release Notes' section with a specific syntax to add release notes to a PR. The idea behind this is that these release notes are automatically picked up on release and added to the release notes of the release.

That doesn't seem to work.

Example:
This release contains the PRs #66, #71, #79, and #80. All of them specify multiple release notes in the corresponding section, but none of these appear in the actual release notes of the mentioned release.

[Diaphteiros]

I'm wondering whether there is something broken or simply not implemented.

I know that the Gardener pipeline, which we used before, had some kind of logic to collect release notes in exactly this fashion.
The question is: Did we simply overtake the Gardener PR template, but didn't implement anything to actually replace the Gardener pipeline's functionality? Or do we have some logic in place which is supposed to collect the release notes, but just doesn't work?

In either case, collecting release notes from PRs is a problem that has probably already been solved multiple times and I would expect Github Actions to be available which can do this kind of stuff.

[In-Ko]

1. Is this something we need to refine ? I don't think so honestly.
2. @moelsayed was implementing the release note generation stuff, so maybe he could shed some light here and help ?

[In-Ko]

As discussed today: Let's rework the release note generation a little bit, so that also the [DEVELOPER] and [OPERATOR] notes are included.