Ruby 3.1 support

[cben]

• it's not currently in test matrix (ruby-head is 3.2-to-be but not treating it as blocker)
• Locally with 3.1.0 on linux, bundle install works but tests fail with:

  File does not exist: net/smtp
  

  Apparently spun out to 'net-smtp' gem. Not sure why we depend on it (real fix probably belongs in some library we depend on that imports it).
• Tests print many deprecations.

[cben]

Currently the only deprecations message I see in 3.1 runs is:

/Users/runner/hostedtoolcache/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/httpclient-2.8.3/lib/httpclient/ssl_config.rb:370: warning: assigned but unused variable - pathlen
/Users/runner/hostedtoolcache/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/httpclient-2.8.3/lib/httpclient/ssl_config.rb:51: warning: method redefined; discarding old initialize
/Users/runner/hostedtoolcache/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/httpclient-2.8.3/lib/httpclient/ssl_config.rb:58: warning: method redefined; discarding old add_cert
/Users/runner/hostedtoolcache/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/httpclient-2.8.3/lib/httpclient/ssl_config.rb:58: warning: method redefined; discarding old add_file
/Users/runner/hostedtoolcache/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/httpclient-2.8.3/lib/httpclient/ssl_config.rb:58: warning: method redefined; discarding old add_path

which is nothing we can fix here. I guess I saw more in the past and they got fixed in various gems? 🤷