Make examples Java 11 compatible with IntelliJ (#3712)
Make examples Java 11 compatible with IntelliJ (#3712)

IntelliJ fails to compile example projects because of a wrong target
bytecode version. It marks the target bytecode of examples as Java 8.
We allow to use Java 11 for examples.

Modification:

- Set 11 to `sourceCompatibility` and `targetCompatibility` for examples

Result:

Compile Armeria project with IntelliJ built-in Java compiler
minwoox/armeria,trustin/armeria,minwoox/armeria,minwoox/armeria,line/armeria,kojilin/armeria,kojilin/armeria,kojilin/armeria,trustin/armeria,line/armeria,line/armeria,trustin/armeria,kojilin/armeria,line/armeria,trustin/armeria,line/armeria,line/armeria,minwoox/armeria,kojilin/armeria,minwoox/armeria,trustin/armeria,kojilin/armeria,minwoox/armeria,trustin/armeria