Use modern SSLContext API

[michaelklishin]

It's already possible to post-configure TLS context using
Bunny::Connection#transport and Bunny::Transport#configure_tls_context
but since we already require Ruby 2.5, using a more modern
API makes sense. It also allows the user to set TLS version to 1.3 [1].

Note that TLS 1.3 support is an all-or-nothing proposition
because there are no shared cipher suites with earlier versions,
by design.

While at it, drop SSLv3 support because it's been compromised and deprecated for so many years,
many runtimes no longer support it as an option.

References #628.

1. https://rabbitmq.com/ssl.html#tls1.3

[michaelklishin]

With a RabbitMQ 3.9 node on Erlang 24.3 with the following rabbitmq.conf:

#
# TLS
#

listeners.ssl.default  = 5671
ssl_options.cacertfile = /path/to/tls-gen.git/basic/result/ca_certificate.pem
ssl_options.certfile   = /path/to/tls-gen.git/basic/result/server_certificate.pem
ssl_options.keyfile    = /path/to/tls-gen.git/basic/result/server_key.pem

I could not make one Bunny TLS test example modified to use TLS 1.3. The node kept logging

[notice] <0.1324.0> TLS server: In state start at tls_handshake_1_3.erl:2964 generated SERVER ALERT: Fatal - Illegal Parameter

and it's not obvious what that "illegal parameter" is without a traffic dump and extra digging I cannot do right now.

But all existing TLS 1.2 tests pass, so that's a step in the right direction.