[Maintenance] Move PMD configuration to refarch-pmd library

[devtobi]

Relevant template

refarch-backend, refarch-eai

Problem description (optional)

We decided to centralize tool related configurations into github.com/it-at-m/refarch under the path refarch-tools. For now we only introduce the PMD configuration file as a separate maven module. We should migrate our PMD configuration to this new module. This way we only need to maintain the file in a single place.

Desired solution

• Add refarch-pmd as a Maven dependency
• Update the PMD configuration in pom.xml to use the new dependency (see https://maven.apache.org/plugins/maven-pmd-plugin/examples/multi-module-config.html#configure-the-other-projects-to-use-it)
• Remove the shared-files folder.

Considered alternatives (optional)

No response

Additional context (optional)

No response

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