Use flatten-maven-plugin when releasing

While looking at https://repo.maven.apache.org/maven2/org/mariadb/jdbc/mariadb-java-client/4.0.0/mariadb-java-client-4.0.0.pom I noticed that there are a lot of unused/irrelevant pieces of XML (e.g. the version of the compiler plugin) that nobody really uses when they add the dependency.

These could be removed utilizing the Flatten Maven Plugin ( https://www.mojohaus.org/flatten-maven-plugin/index.html ).

If you need an example how it can be implemented you can have a look into our repos:
https://github.com/xdev-software/standard-maven-template/blob/1707302b7ed01042a88095cd5901b9727d6b5c44/src/pom.xml#L204-L227