Drop EOL ruby/rspec combinations

[mjankowski]

Summary:

• Drop support for all rubies earlier than 3.2, which is currently the oldest non-EOL ruby. This version will be EOL in a few months, at which point we could drop as well, or just support through 4.1 or whatever.
• Drop support for rspec 3.6 and 3.7, which both have issues on ruby 4.0 and were excluded on CI. These versions are both 8+ years old, with many supported/compatible releases in the meantime, so this is probably safe? I suspect we could drop even more here, but as a first pass just do the ones with interop issues.
• For each ruby version step, bump the rubocop configured version, and apply any autocorrects. This was fortunately pretty minimal with each bump
• Update CI config, gemspecs, Appraisals - to align with previous changes

[mjankowski]

Immediately after opening this I noticed #116 which does the first portion of this.

Rebased after that merged, so this is now just the rubocop bumps and rspec drops.