Configure dependabot

[BrianJKoopman]

Description

Configure dependabot to update the used github-actions and (perhaps more importantly) the crossbar server base docker image.

Motivation and Context

I noticed recently that our crossbar base image is a few years old! This'll help keep us up to date with the latest version released by crossbar.

How Has This Been Tested?

Sort of need to test in the repo live.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• Unless I am preparing a release, I have opened this PR onto the develop branch.