Gaoxy v0.12.1

Gaoxy 0.12.1

Native HTTP/2 uses the documented first-party HTTP.HTTP2.HPACK codec from
http_core >= 0.21.16 instead of HPAX. The integration preserves decoder state,
header bounds and zero-capacity response encoding.

The HTTP/TLS/QUIC dependency cohort advances together to 0.21.16.
The elixir_quic_http3, http_fetch and http_web_socket package requirements
use ~> 0.21 instead of exact pins. The new direct http_core requirement uses
the same compatibility range with a >= 0.21.16 floor.
The unsupported optional flag is removed from the umbrella gaoxy_http3
dependency; the staged gaoxy_test Hex extension remains optional.

All six Hex distributions and the standalone application advance together to
0.12.1.