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.

Also "fix" many TLS test failures due to yet more changes
in the default options for TLS.

Also small changes to make Dialyzer happy.

And ensure that we don't infinite loop when retries are
enabled, by decrementing the retry count instead of using
a new one.

Also check for ssl:negotiated_protocol {error,closed}
which was possible but was not documented in OTP before
this change.

Thanks @voluntas for the help.