Conditionally alias constants for TLSv1.3 to support older OpenSSL releases

[michaelklishin]

See #629, #646.

For environments that do provide a supported Ruby version but do not provide a TLS 1.3-compatible
OpenSSL versions, it would be great to avoid a module load time exception and simply not have
1.3 as a supported version. Conditionally populating the map of TLS aliases in Bunny::Transport
should be enough.

However, a quick investigation suggests that it is not entirely trivial: older OpenSSL module
versions do not provide the same set of constants, and reproducing an environment with, say,
Ubuntu 18.04 and Ruby 2.6 will take extra effort:

• Before Ruby 2.6, OpenSSL::SSL::TLS1_2_VERSION and similar constants were not available
• Ruby 2.6 is not available in standard Ubuntu repositories, they go from 2.5 in 18.04 to 2.7 in 20.04 and then 3.0 in 22.04
• Using defined?(OpenSSL::SSL::TLS1_3_VERSION) should be enough to work around this but to test it I'd have to build Ruby 2.6 on Ubuntu 18.04
• Building Ruby 2.6 on arm64 hosts does not seem to be possible (likely because it pre-dates ARM64 Macs in wide use)