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.

This is to avoid reconnecting forever in a loop on bad
configuration (such as IP in a different subnet).

The number of unacknowledged pings that can be tolerated
before the connection is forcefully closed.

When a keepalive ping is sent to the peer, a counter is
incremented and if this counter exceeds the tolerance limit,
the connection is forcefully closed. The counter is
decremented whenever a ping ack is received from the peer.

By default, the mechanism for closing the connection based
on ping and ping ack is disabled.

Loïc Hoguin: I have edited a lot of the code and renamed
a few things as well as simplified the docs and increased
test timeouts to avoid race conditions.