ci(dependabot): stop filing github-actions updates

why: Action pins in this repository are maintained by a researched sweep that
reads every `uses:` line from trunk, verifies each target tag resolves, and
lands one commit per action with the upstream release notes and the migration
impact recorded in the message. Dependabot's pull requests duplicate that work
with a bot-authored commit message, and the notification volume across the
estate is not worth what they add.

what:
- deleted .github/dependabot.yml (github-actions was its only ecosystem)
- Dependency updates for every other ecosystem continue unchanged

Author: tony