chore: move renovate-config-validator to GH Actions

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

Most of the contributors won't ever update renovate configuration, yet the pre-commit hook takes quite some time to install locally (at least for me).

So instead of running it through pre-commit, this PR moves the workflow to a dedicated GitHub Actions workflow, similarly to the one we have for codecov configuration.

The PR also updates the schedule for lock file maintenance, so it runs on Saturday, same as the other updates, instead of Monday (which also allows validating the changes in the PR, to confirm that the workflow works correctly).

[codecov-commenter]

Codecov Report

Merging #233 (a82c018) into main (093e55c) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #233   +/-   ##
=====================================
  Coverage   95.7%   95.7%           
=====================================
  Files         32      32           
  Lines        914     914           
  Branches     203     203           
=====================================
  Hits         875     875           
  Misses        26      26           
  Partials      13      13           

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