Remove usages of `ByteBufferCallbackAccumulator`

**Jetty version(s)**
12.1.x

**Enhancement Description**
`ByteBufferCallbackAccumulator` has been deprecated but there still are places where we use it. We should replace all those usages with `RetainableByteBuffer.DynamicCapacity` and eventually remove this class.

See: #11801

Start by reviewing issue #11801 and searching the Jetty 12.1.x codebase for usages of ByteBufferCallbackAccumulator. Replace each usage with RetainableByteBuffer.DynamicCapacity, then verify that no usages remain and that ByteBufferCallbackAccumulator can be removed. The payload names no specific files or tests.

Written by the indexing model from the issue text.