Make sure changelog content changed and has valid format

[BHuber-PlanB]

Is your feature request related to a problem? Please describe.
No, it is not.

Describe the solution you'd like
The PR Validation workflow should make sure the changelog file's content has changed and has expected content format.

Additional context
Changelog
PR validation workflow
Format:

## Added

- ...
- ...
- ...

## Changed

- ...
- ...
- ...

## Fixed

- ...
- ...
- ...

## Removed

- ...
- ...
- ...

[Link to GitHub issue](https://github.com/PlanBGmbH/flinkey-web-components/issues/<issue-number>)