Previously, the custom layout sample did not configure the source
and target for the compiler. This meant that both defaulted to 1.5
which is not supported by Java 9's javac. The oldest support version
is 1.6.

This commit sets the maven.compiler.source and maven.compiler.target
properties to 1.8 (the lowest version support by Boot 2.0) allowing
the sample to build with both Java 8 and Java 9.

See gh-10515