feat: enable Renovate

What is your Merge about?

Add renovate.json to the project to enable automatic image digest updates via the Renovate bot.

Related issues:

• https://repo1.dso.mil/dsop/canonical/ubuntu/ubuntu-pro-cis-stig-20.04/-/issues/1#repository-requirements
• https://repo1.dso.mil/dsop/dccscr/-/issues/1995

Closes https://repo1.dso.mil/dsop/canonical/ubuntu/ubuntu-pro-cis-stig-20.04/-/issues/13