#35 Migrate from Gradle to Maven

It is a big hassle to get Gradle releases work in GH actions as
we like. After spending some time fiddling around with Gradle,
we decidedto use god oldMaven, we have more knowledge in.

Signed-off-by: Sven Strittmatter <sven.strittmatter@iteratec.com>

Author: Weltraumschaf