Dependabot tweaks

[divergentdave]

This makes three changes to our Dependabot configuration:

• Ignore all updates to the library "postgres" container image. Putting this in the configuration will avoid some back-and-forth with each new release branch.
• Group all GHA reusable actions related to Docker together.
• Ignore all semver-minor updates to libprio-rs. Since these are often wire-breaking changes, these tend to be just noise. We can handle updates in other cases manually.