CI: Enable dependabot for GitHub Actions

[akihironitta]

What does this PR do?

Avoids using very outdated actions.
For example, we currently use actions/setup-python@v2 where the stable one is actions/setup-python@v4.

Does your PR introduce any breaking changes? If yes, please list them.

None

Before submitting

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• Did you make sure your PR does only one thing, instead of bundling different changes together?
• [n/a] Did you make sure to update the documentation with your changes? (if necessary)
• [n/a] Did you write any new necessary tests? (not for typos and docs)
• [n/a] Did you verify new and existing tests pass locally with your changes?
• [n/a] Did you list all the breaking changes introduced by this pull request?
• [n/a] Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
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Did you have fun?

Make sure you had fun coding 🙃

cc @carmocca @akihironitta @Borda