ci: add dependabot workflow to update github actions

[sappelhoff]

following what we have in mne-python: https://github.com/mne-tools/mne-python/blob/562f4730dd3d462abb4ec134270c7cf0a9d53b0e/.github/dependabot.yml#L1

Merge checklist

Maintainer, please confirm the following before merging:

• All comments resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• PR description includes phrase "closes <#issue-number>"

[codecov]

Codecov Report

Merging #1015 (107dad5) into main (a73fc5a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1015   +/-   ##
=======================================
  Coverage   93.90%   93.90%           
=======================================
  Files          25       25           
  Lines        3774     3774           
=======================================
  Hits         3544     3544           
  Misses        230      230           

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