Make JavaDoc respect JPMS module definitions

[mtf90]

With #4559, the checker-qual JAR provides a module-info.java description to be used by the Java platform module system (JPMS). However, its use is currently limited to the compiled .jar artifact. When generating JavaDoc for a module-aware, third-party project that uses checkerframework's annotations, the javadoc tool issues the following warning:

javadoc: warning - The code being documented uses modules but the packages defined in https://checkerframework.org/releases/3.48.3/api/ are in the unnamed module.

The problem is that the documentation is generated in a non-modular context ("java 8 mode") because the build process depends on some gradle-magic to resolve cyclic dependencies between checker-qual (at least in its documentation) and the checker implementation.

To resolve this issue, it should suffice to build the JavaDoc with a module-aware language level as well. However, this would require refactoring the documentation (and maybe more?) to resolve the cyclic dependencies which cannot exist between modules.