Make JavaDoc respect JPMS module definitions

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.

Start by examining the JavaDoc build process and the Gradle configuration that resolves the checker-qual and checker implementation dependency cycle. Compare the current Java 8-mode documentation setup with the module-aware JPMS requirements described in the issue. Done means JavaDoc generation uses module-aware settings without cyclic dependency failures and no longer emits the unnamed-module warning.