Update maven-compiler-plugin for increased JDK compatibility

The maven-compiler-plugin is referenced in the wrong clause of the pom file.
It should be within <project><build><pluginManagement><plugins>.
(See for instance https://maven.apache.org/plugins/maven-compiler-plugin/usage.html )

Otherwise you will see Nullptr Exceptions for instance with JDK11.

For now I propose to only move the plugin and use current version 3.8.0. Increasing the errorprone module version will trigger a lot of diagnostics in errorprone plugins, which should be dealt seperately.

Doublechecked: Still compiles with java 1.8