Add monthly GitHub Actions Dependabot updates

[brianhdk]

What changed

• Add or standardize Dependabot version updates for GitHub Actions.
• Run the update check monthly at 07:00 Europe/Copenhagen.
• Group minor and patch action updates while keeping major updates in separate pull requests.
• Preserve existing Dependabot configuration for other package ecosystems.

Why

This gives repositories with GitHub Actions a consistent, low-noise maintenance cadence while keeping potentially breaking major action upgrades isolated for review.

Impact

Dependabot will check GitHub Actions references monthly. No application or runtime behavior changes.

Validation

• Parsed the resulting configuration as YAML.
• Verified one GitHub Actions update block per repository with the intended schedule and grouping.
• Ran git diff --cached --check.
• Application builds and tests were not run because only .github/dependabot.yml changed.