Compatibility with JDK 16

[sbrannen]

Along the lines of #24517, this issue aims to track JDK 16 compatibility.

For the time being (i.e., during the 5.3.x time frame), we'll simply set up a CI build compiling and executing tests against JDK 16 early access builds.

At this time, however, we are waiting on support for JDK 16 in the following.

• ASM
• Gradle: Support Java 16 gradle/gradle#13481 and Presence of record in src/test/java causes error during execution of JUnit 4 gradle/gradle#14714

[sbrannen]

The upgrade to ASM 9 has already been addressed in #24872.