Resolve "Enable ci pipeline for ecmr-backend"

Related issues

"Closes #21 (closed)"

Acceptance criteria

• Implemented code changes don't diverge from the related issue.
• Version number is incremented according to SemVer. (Snapshot versions are not merged/released.)
• Only released versions of dependencies are used (i.e. no snapshot versions).
• Pipeline still works.
• GitLab issue is up-to-date.

If necessary, the following sections have been updated:

• Readme.md

Proposed squash commit message

Enable ci pipeline for ecmr-backend

• Add jacoco plugin (latest version 0.8.11) to pom.xml to generate test reports
• Add license-header to all java classes
• Use maven:3.9.6-eclipse-temurin-21 as ci pipeline base image-