Since quicer, which provides the QUIC implementation,
is a NIF, Gun cannot depend directly on it. In order
to enable QUIC and HTTP/3, users have to set the
GUN_QUICER environment variable:

  export GUN_QUICER=1

Gun is now tested using GitHub Actions. As a result
OTP-24+ is now required. In addition, the number
of OTP releases tested has been reduced; only the
latest of each major version is now tested.

This also updates Erlang.mk.