GHSA-gq9p-f254-h286: Http4s: Ember HTTP/2 buffers a frame's declared payload before checking SETTINGS_MAX_FRAME_SIZE

### Summary An unauthenticated peer can make Ember's HTTP/2 read loop hold 16 MiB of a single frame in memory on a connection where Ember advertised a 16 KiB limit. The declared length is readable from the frame's first 9 bytes, but it is not compared against SETTINGS_MAX_FRAME_SIZE until the whole payload has been read into a contiguous buffer. That is 1024x amplification per connection, bounded by the protocol's 24-bit length field rather than by any setting, and at the default maxConnections of 1024 it puts about 16 GiB of heap in reach. …