Update pom.xml to use java 11

[donbourne]

Trying to build this project with JDK 8 currently fails due to certain dependencies having been built with Java 11. While it's technically possible to build with ...

    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>

... it makes more sense to build with ...

    <maven.compiler.source>11</maven.compiler.source>
    <maven.compiler.target>11</maven.compiler.target>

... so that people realize immediately that they need at least JDK 11 to build the project.