Set maven-assembly-plugin version 3.1.1 explicitly for each POM module

Set maven-assembly-plugin version 3.1.1 explicitly for each POM module

Background: There are CI tests failing because the version is not taken
from the parent POM, even if set in the 'pluginManagement' section.
Instead, it is resolved via the Maven Super POM, e.g. 2.2-beta-5, see:
https://maven.apache.org/ref/3.6.3/maven-model-builder/super-pom.html
That assembly plugin version in turn requires plexus-archiver
1.0-alpha-12. The latter cannot be downloaded from Maven Central,
leading to the problems seen during builds.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>