Bump minimum Ruby version (#18294)

Mastodon has been incompatible with Ruby 2.5 for a few releases due to
some dependencies, this just updates the Gemfile description to match
that situation.

Gemfile

 # frozen_string_literal: true
 
 source 'https://rubygems.org'
-ruby '>= 2.5.0', '< 3.1.0'
+ruby '>= 2.6.0', '< 3.1.0'
 
 gem 'pkg-config', '~> 1.4'
 gem 'rexml', '~> 3.2'