Use newer minimal java source and target version

To be able to compile the project with java versions greater than 1.8
a minimal java version of at least 1.8 is needed. Change the minimal
java source and target version and use a minimal java compiler version
1.8 to deal with that.

Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>
Reviewed-by: Bastian Germann <bage@linutronix.de>