[Maintenance] Change pinning of RedHat base images

[devtobi]

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

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