[MPOM-342] Configure some default excludes for apache-rat-plugin

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MPOM-342](https://issues.apache.org/jira/browse/MPOM-342?redirect=false)** and commented

As the version 0.14 now also checks for license headers in `.gitignore` and `.gitattributes` files (compare with RAT-308) and these files seem to be considered not relevant (compare with https://www.apache.org/legal/src-headers.html#faq-exceptions and https://github.com/apache/maven-resolver/pull/193#issuecomment-1249435233) those should be globally ignored for all ASF projects.

This has already been done for some downstream projects like https://github.com/apache/maven/pull/792/files#diff-9c5fb3d1b7e3b0f54bc5c4182965c4fe1f9023d449017cece3005d3f90e8e4d8R578.

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**Issue Links:**
- [RAT-308](https://issues.apache.org/jira/browse/RAT-308) After update from 0.13 to 0.14 check emits violation for ".gitattributes"
(_**"is caused by"**_)

Start by locating the apache-rat-plugin configuration in the Maven parent POM and compare the downstream Maven change referenced in the issue. Done means .gitignore and .gitattributes are globally excluded from Apache RAT license-header checks.

Written by the indexing model from the issue text.