ci: set_assignee: add maintainers before collaborators if there are too many reviewers

[maass-hamburg]

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

When a PR is submitted, that changes things in multiple different areas, the amount of potential reviewers might exceed 15 reviewer, so that not all potential reviewers are set and notified. Due to the fact that maintainers and collaborators are grouped together, it can happen that maintainers are not added as reviewers, even thew there a changes in their area.

Describe the solution you'd like

Change that the set_assignee script so, that the adding of reviewers and that of assigners is independent.
On adding reviewers first add all maintainers of the matched areas and add collaborators after it, if there is still room.

Maybe it is even possible that the zephyr-bot mentions all potential reviewers, that did not got added because of the limit

Describe alternatives you've considered

No response

[github-actions]

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