EOLing Ruby 2.5 Support in 3.2.x

[petergoldstein]

The 3.2.x releases of Dalli will no longer support Ruby 2.5 - they will be restricted to Ruby 2.6 and up.

I had originally intended to EOL Ruby 2.5 support in 3.2.0 I forgot to include the relevant changes. So the changes (Gemspec, CI, etc.) will be released as part of the 3.2.1 release.

Thanks!