Add a build profile for JDK 16 (#10676)
Add a build profile for JDK 16 (#10676)

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.

Result:
Netty now builds on JDK 16 pre-releases (provided they've not broken compatibility in some way).
ejona86/netty,Spikhalskiy/netty,zer0se7en/netty,doom369/netty,johnou/netty,artgon/netty,johnou/netty,zer0se7en/netty,tbrooks8/netty,NiteshKant/netty,ejona86/netty,johnou/netty,artgon/netty,netty/netty,andsel/netty,artgon/netty,artgon/netty,zer0se7en/netty,zer0se7en/netty,johnou/netty,andsel/netty,doom369/netty,johnou/netty,ejona86/netty,NiteshKant/netty,tbrooks8/netty,netty/netty,netty/netty,netty/netty,Spikhalskiy/netty,zer0se7en/netty,andsel/netty,doom369/netty,Spikhalskiy/netty,netty/netty,tbrooks8/netty,doom369/netty,andsel/netty,NiteshKant/netty,Spikhalskiy/netty,artgon/netty,ejona86/netty,Spikhalskiy/netty,NiteshKant/netty,tbrooks8/netty,andsel/netty,doom369/netty,tbrooks8/netty,NiteshKant/netty,ejona86/netty