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, subproject netty-zlib maintains a copy of com.jcraft.zlib
- JCTools uses sun.misc.Unsafe, not VarHandles, subproject netty-jctools maintains a copy of jctools
- PlatformDependent uses sun.misc.Unsafe. Tests run successful by -add-exports=jdk.unsupported/sun.misc=ALL-UNNAMED and
  --add-opens=jdk.unsupported/sun.misc=ALL-UNNAMED
- finalize() calls in PoolThreadCache, PoolArena are deprecated
- when using boringssl static, setting of SSLContent.useTasks to false makes tests hang. Tests using false setting were removed.

Subproject organization

Original netty subproject names are not related to package names. I reorganized the names to allow better assignment
between subproject name, package name, artifact names, and java module. The following reorgnizations were performed:

all -> [todo]
bom -> [todo]
buffer -> netty-buffer
codec -> netty-handler-codec, netty-handler-codec-compression, netty-handler-codec-protobuf
codec-dns -> [todo]
codec-haproxy -> [todo]
codec-http -> netty-handler-codec-http, netty-handler-codec-rtsp, netty-handler-codec-spdy