The following changes were performed on the original source code:

- removed slf4j, log4j, log4j2 logging
- removed internal classes for GraalVM (SCM)
- removed internal classes for Blockhound
- removed jetbrains annotations
- private copy of jctools in io.netty.jctools
- removed SecurityManager code
- add module info
- removed lzma dependency (too old for module)
- use JdkZLibDecoder/JdkZlibEncoder in websocketx
- removed JettyAlpnSslEngine
- removed JettyNpnSslEngine
- removed NPN
- use of javax.security.cert.X509Certificate replaced by java.security.cert.Certificate
- private copy of com.jcraft.zlib in io.netty.zlib
- precompiled io.netty.util.collection classes added
- refactored SSL handler to separate subproject netty-handler-ssl
- refactored compression codecs to separate subproject netty-handler-codec-compression
- moved netty-tcnative/openssl-classes to netty-internal-tcnative
- removed logging handler test
- removed native image handler test

Challenges for Netty build on JDK 21

- unmaintained com.jcraft.jzlib
- JCTools uses sun.misc.Unsafe, not VarHandles
- PlatformDependent uses sun.misc.Unsafe
- finalize() in PoolThreadCache, PoolArena