ci: replace dead labels tool with ghaction-github-labeler

[MartinHjelmare]

Description of change

The labels PyPI tool (hackebrot/labels) crashes against the current
GitHub API with:
TypeError: Label.init() got an unexpected keyword argument 'archived_at'

Replace it with crazy-max/ghaction-github-labeler v6.0.0 (actively
maintained, native action). Convert .github/labels.toml to the YAML
format the new action expects and remove the now-unused .toml file.

Pull-Request Checklist

• Code is up-to-date with the main branch
• This pull request follows the contributing guidelines.
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• Documentation has been updated to reflect this change
• The new commits follow conventions outlined in the conventional commit spec, such as "fix(api): prevent racing of requests".

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• If CI / test is failing, try uv run pytest for further information.