Motivation:
Java 16 will come around eventually anyway, and this makes it easier for people to experiment with Early Access builds.
Modification:
- Added Maven profiles for JDK 16 to relevant pom files.
- Removed the `--add-exports java.base/sun.security.x509=ALL-UNNAMED` argument when running tests; we've not needed it since the Java11-as-baseline PR landed.
Result:
Netty now builds on JDK 16 pre-releases (provided they've not broken compatibility in some way).
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