Add dependabot for Github Actions

[Rotzbua]

Change

Github Actions maintenance must be done manually by inspecting the GH action page and compare it with your current workflows. This can be done by the dependabot.

To avoid the well known dependabot PR spam:

• only run for GH Actions
• the bot only runs once a month
• found updates will be grouped to one single PR.

Why maintance for GH Actions required?

They run on node and will be deprecated and stop working after time. https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/

Benefit

Avoids manual chore like #199

Disadvantage

The dependabot does not migrations. So still read changelog ;-)