Require Ruby >= 3.1

[mhashizume]

Currently, Facter requires Ruby >= 2.5 in order to be compatible with Puppetserver 7, older versions of which use JRuby 9.2.y.z (which targets compatibility with MRI 2.5).

We have attempted to drop Ruby 2.5 support in #2674, but encountered compatibility issues with those older versions of Puppetserver and had to revert in #2686

After the Puppet 7 series has been retired, we should bump up Facter’s minimum Ruby version to 3.1 to match with Puppet and Puppetserver 8.

We may want to wait until a new major Facter version to do this, although many projects drop compatibility with older versions of Ruby in minor releases (see Bundler, Nokogiri, AWS SDK, and more).