[Fix] Enforce Java 1.8 API

[folmate]

Current version only enforces that the source code syntax and target byte code is compliant with Java 8, but it does not enforce that the used APIs are also compatible.

Example

Optional<String> foo = Optional.of(null);
foo.isEmpty(); // Introduced in Java 11

This code compiles, but results in a runtime failure if the Java JRE is <11.

The patch prevents the compilation of code, where newer API is used.