Enable dependabot for github actions

[ppawlowski]

Description

This pull request enables dependabot for github actions.

Related Issue(s)

https://github.com/FlowFuse/CloudProject/issues/358

Checklist

• [x] I have read the contribution guidelines
• [ ] Suitable unit/system level tests have been added and they pass
• [ ] Documentation has been updated
  • [ ] Upgrade instructions
  • [ ] Configuration details
  • [ ] Concepts
• [ ] Changes flowforge.yml?
  • [ ] Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • [ ] Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• [ ] Includes a DB migration? -> add the area:migration label

[Steve-Mcl]

Is dependabot really necessary?

This repo is 100% standalone (no deps)

[ppawlowski]

Is dependabot really necessary?

This repo is 100% standalone (no deps)

This PR enables dependabot for GitHub Actions only. Since workflows in this repo use external actions, I would like to track their latest available versions with dependabot.

[Steve-Mcl]

Approved. I'll let you chose a merge window @ppawlowski