Support building with Java 21, drop Java 7

[alexei-osipov]

Currently an attempt to build the DFP project with Java 21 result in error:

> Task :java:dfp:compileJava FAILED
error: Source option 7 is no longer supported. Use 8 or later.
error: Target option 7 is no longer supported. Use 8 or later.

This is caused by remaining support of Java 7 for DFP.

compileJava {
    sourceCompatibility = 1.7
    targetCompatibility = 1.7
}

We should support building at on least one LTS java version: Java 17 or Java 21. I think we can drop support of Java 7 for simplicity.