[CI] Add cooldown configuration to Dependabot

[thisisnic]

Describe the enhancement

The Dependabot configuration in .github/dependabot.yml is missing a cooldown configuration for the github-actions package ecosystem. Adding a cooldown prevents Dependabot from flooding the repo with PRs when multiple actions release updates simultaneously.

Component(s)

Continuous Integration

[thisisnic]

We don't need this, we review updates carefully enough that it isn't a risk