feat(ci): configure 7-day cooldown for Dependabot GitHub Actions updates

[okurz]

Motivation:
Reduce CI pipeline load and review fatigue by pacing automated updates.

Design Choices:
Configure cooldown with default-days set to 7 in the github-actions section
of .github/dependabot.yml.

Benefits:
Paces automated pull requests to match the established openQA and qem-dashboard
update patterns.

Related issue: https://progress.opensuse.org/issues/203049

[AdamWill]

LGTM. Looking at the upstream docs, it seems like there is a default three-day cooldown but it "does not apply to security updates", which makes it useless for safety purposes (if you're trying to get an evil update rolled out, just declare it's a security update). The docs never elaborate on this, though - they never state whether an explicitly-configured cooldown period includes security updates, or document any kind of "include security updates in cooldown" setting. So I hope explicitly-configured cooldown does include "security updates". Still, this appears to be the best we can do.