ci: bump github actions versions

[afuetterer]

What does this PR do?

This PR bumps the GitHub Actions versions and via

- uses: actions/setup-python@v6
  with:
    python-version: "3.13" # <-- added this part

removes the warning that was displayed in the GHA interface, e.g.

lint
The python-version input is not set. The version of Python currently in PATH will be used.

Ref: https://github.com/MaartenGr/BERTopic/actions/runs/18008728788

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• This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
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• Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes (if applicable)?
• Did you write any new necessary tests?