Add Dependabot for Github actions

[pjonsson]

Dependabot will submit pull requests when packages get new versions released. After merging this, Dependabot should open pull requests that resolve the warnings for the workflows in the actions tab, as seen here for example:

https://github.com/TerriaJS/TerriaMap/actions/runs/8150265114

Link to Dependabot documentation:

https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot