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.

[agdavydov81]

Yes, the current master branch supports only Java 18 or 19 for jar compilation.
But this restriction is only about build environment and do not related to the runtime environment.
Yes, I know about Java 18 and 19 EOL, but I have a questions:

1. Do our all (without exceptions) customers support Java 8 and ready for migration?
2. What the profit we will get from this migration? I mean what features to the final product will be added?

I'm not against the migration, but right now I see only the Java 7 version dropping without any benefits.

[alexei-osipov]

1. Yes. All of them. There are no actively supported customers on Java 1.7. Even more, currently all projects on Java 11 at least.
2. First of all, this improves project maintainability. Secondly, performance. Having Java 1.7 as target reduces the set of API's and optimizations that Java compiler can use.

[agdavydov81]

After R&D discussion this issue was resolved positively.
#102