Resolve 6.1 and 6.0 divergence

[yaauie]

As discovered in #50, we have a divergence between the 6.1-series of this plugin (which requires Logstash 8+), and 6.0-series (which runs on Logstash 7+).

In the above-mentioned PR, we back-ported some fixes from the 6.1-series into what is hopefully a temporary 6.0.x maintenance branch, and also identified and fixed other bugs that exist in the 6.1-series on main.

• forward-port baseline specs
• forward-port server-mode partial writes fix
• forward-port fix to server-mode plugin close and addition of client-mode plugin close
• forward-port fix to logging to use Socket#peer instead of Socket#to_s, as-cached as early as possible in any scope to prevent post-socket-close issues
• forward-port trace-level logging around partial writes in both client and server modes
• forward-port changelog entry for 6.0.3

In addition, it would be good to see if we can relax the dependency of the 6.1-series to allow it to run on still-supported versions of Logstash 7.x so that we can avoid maintaining two somewhat-significantly diverged branches of this plugin (even if doing so makes using the new ssl_supported_protocols it introduces a configuration error when run on Logstash 7).