Use Java 8 source and target in Maven

Building the jgit aggregator with Maven leads to a compiler level 1.6
for me, thereby failing the build. Multiple profiles specify
source/target level separately, but there seems no global setting.
Adding the global properties makes the jgit aggregator compile on my
system.

Change-Id: Ia5613cc0fcf6085265a9e48f940e5a7d3f998608
Signed-off-by: Michael Keppler <Michael.Keppler@gmx.de>

pom.xml

     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
     <maven.build.timestamp.format>yyyyMMddHHmm</maven.build.timestamp.format>
+    <maven.compiler.source>1.8</maven.compiler.source>
+    <maven.compiler.target>1.8</maven.compiler.target>
     <bundle-manifest>${project.build.directory}/META-INF/MANIFEST.MF</bundle-manifest>
 
     <jgit-last-release-version>5.4.0.201906121030-r</jgit-last-release-version>