update actions/checkout and introduce dependabot

[madrob]

Update the actions/checkout action to the latest release (v6.0.1)

Motivation:

https://github.com/actions/checkout?tab=readme-ov-file#whats-new

• Improved credential security: persist-credentials now stores credentials in a separate file under $RUNNER_TEMP instead of directly in .git/config
• No workflow changes required — git fetch, git push, etc. continue to work automatically
• Running authenticated git commands from a Docker container action requires Actions Runner v2.329.0 or later
• Updated to the node24 runtime
  This requires a minimum Actions Runner version of v2.327.1 to run.

Modifications:

Updated the dependency version and added dependabot to take care of future upgrades.

Result:

Our workflows will be more secure!