[github-actions] add Dependabot configuration for GitHub Actions updates (#8418)
[github-actions] add Dependabot configuration for GitHub Actions updates (#8418)

Add a Dependabot configuration that checks once a week if the GitHub
Actions are still using the latest version. If not, it opens a PR to
update them.

It will actually open few PRs, since only major versions are specified
(like v3), so only on a major release (like v4) it will update and
open a PR. But it helps actively keep GitHub Actions workflows up to
date and secure.
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