Update pom.xml to use maven.compiler.release

[aitseitz]

• changed to <maven.compiler.release>
• updated maven.compiler.version to 3.14.0

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CONVINCING DESCRIPTION

#237

RELATED INFORMATION

https://maven.apache.org/plugins/maven-compiler-plugin/examples/set-compiler-release.html

Fixes #

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[AFaust]

Around line ~415 the build -> pluginManagement -> plugins -> plugin defintion for the maven-compiler-plugin refrences the removed source/target properties. This was done in a past Maven version where - for reasons unknown - the properties alone did not work. These references should be removed as well as the properties themselves. Otherwise I have checked and verified that the class file version remains unaffected by this change and still marks compiled code as Java 8 compatible (class file version 52).

[AFaust]

Minor change in pom.xml requested to remove reference to the removed properties.

[aitseitz]

@AFaust
Absolutly right.

I changed it to adress JDK8 as well as JDK9 and above regarding to
https://maven.apache.org/plugins/maven-compiler-plugin/examples/set-compiler-release.html#usage-on-jdk-8

[AFaust]

Why not use the maven.compiler.release property here? Or delete that in the properties section - either way would be fine.

I rebased the PR and applied requested change myself.