Configure Dependabot to check GitHub Actions for RAP repos

[suzannehamilton]

Some of the RAP repos don't have Dependabot configured to keep the GitHub Actions workflows up to date.

For example, job-runner is configured to update GHA. But the r-docker repo is not.

We should check all the RAP repos, and configure Dependabot to update GHA workflows.

[suzannehamilton]

@madwort I made a note to create this ticket a few weeks ago, when we had a chat about dependency PRs. Have I missed anything?

[rebkwok]

Repos that do check GHA with dependabot:

• airlock
• job-runner
• ehrql
• pipeline
• bennettbot
• backend-server
• tpp-database-util
• osgithub
• python-docker
• sqlrunner
• stata-docker
• openprescribing
• update-dependencies-action
• kissh

Repos that don't check GHA with dependabot:

• base-docker
• r-docker
• proxy
• sysadmin