[MPOM-375] Add license header automatically to all files

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MPOM-375](https://issues.apache.org/jira/browse/MPOM-375?redirect=false)** and commented

Currently license headers are only added to some java files. One should leverage option https://github.com/diffplug/spotless/tree/main/plugin-maven#license-header to add/modify it in all files (also to always properly reflect the year of the last modification).

Also e.g. Java files within `src/test/resources` are currently not considered with regards to license file.

This is currently blocked though by https://github.com/diffplug/spotless/issues/1418.

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**Affects:** MAVEN-38

**Issue Links:**
- [MPOM-349](https://issues.apache.org/jira/browse/MPOM-349) Enable spotless for autoformatting

Start by reviewing the Spotless Maven license-header option and the linked Spotless issue 1418, then inspect the existing license-header configuration referenced by the parent POMs. Done means license headers are added or updated for all files, including Java files under src/test/resources, with the last-modification year reflected correctly once the external blocker is resolved.

Written by the indexing model from the issue text.