[Maintenance] Change pinning of RedHat base images

### Relevant component

refarch-tools

### Problem description (optional)

RedHat recently changed their tagging strategy for the ubi9/nginx and ubi9/openjdk images. Now there are minor tags available, which we could use to decrease the amount of PRs we have to deal with. This way we still get security related updates and remove the burden to update often from the projects.
This however requires that we define a custom renovate rule, as we still want to use patch level pinning for all other docker-related versions.

### Desired solution

Add custom packageRule to the refarch-renovate configuration, which sets pinning strategy to minor.

### Considered alternatives (optional)

Keep as is and require patch level pinning for base images

### Additional context (optional)

_No response_

### No duplicate

- [X] I confirm that this issue is not a duplicate

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct

Locate the refarch-renovate configuration and review its existing Docker-related Renovate rules, focusing on ubi9/nginx and ubi9/openjdk. Add the custom packageRule described in the issue and verify that those images use minor-level pinning while other Docker versions retain patch-level pinning.

Written by the indexing model from the issue text.