Remove usages of ByteBufferPool.Accumulator

[lorban]

Jetty version(s)
12.1.x

Enhancement Description
ByteBufferPool.Accumulator is now deprecated and should be replaced with better design, like RetainableByteBuffer.DynamicCapacity. Both the HTTP3 generators and FCGI still rely on this class.

Once its usages are gone, we should consider removing it entirely.

See: #11801

[gregw]

Deprecated and all usages have been removed. Remove class when this issue goes stale or after a few releases.