Introduce support for Netty 5 Buffer

This commit introduces support for Netty 5's Buffer, in the form of
Netty5DataBuffer. Because of the new API offered by Buffer, several
changes have been made to the DataBuffer API:

- CloseableDataBuffer is a simpler alternative to PooledDataBuffer, and
  implemented by Netty5DataBuffer. DataBufferUtils::release can now
  handle CloseableDataBuffer as well as PooledDataBuffer.
- PooledDataBuffer::touch has been moved into a separate interface:
  TouchableDataBuffer, which is implemented by Netty5DataBuffer.
- The capacity of DataBuffers can no longer be reduced, they can only
  grow larger. As a consequence, DataBuffer::capacity(int) has been
  deprecated, but ensureWritable (formally ensureCapacity) still exists.
- DataBuffer::slice and retainedSlice have been deprecated in favor of
  split, a new method that ensures that memory regions do not overlap.
- DataBuffer::asByteBuffer has been deprecated in favor of toByteBuffer,
  a new method that returns a copy, instead of shared data.
- DataBufferFactory::allocateBuffer has been deprecated in favor of
  allocateBuffer(int).

Closes gh-28874

src/docs/asciidoc/core/core-databuffer-codec.adoc

@@ -125,7 +125,7 @@ release it immediately, it can do so via `DataBufferUtils.release(dataBuffer)`.
 . If a `Decoder` is using `Flux` or `Mono` operators such as `flatMap`, `reduce`, and
 others that prefetch and cache data items internally, or is using operators such as
 `filter`, `skip`, and others that leave out items, then
-`doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release)` must be added to the
+`doOnDiscard(DataBuffer.class, DataBufferUtils::release)` must be added to the
 composition chain to ensure such buffers are released prior to being discarded, possibly
 also as a result of an error or cancellation signal.
 . If a `Decoder` holds on to one or more data buffers in any other way, it must