Make the extension compatible with Ruby 3

Ruby 2 has been end-of-lifed. There doesn't seem to be a specific reason
for this Ruby version requirement, so let's relax it.

solidus_recently_viewed.gemspec

@@ -10,7 +10,7 @@ Gem::Specification.new do |s|
   s.summary     = 'Adds recently viewed products to Spree'
   s.description = s.summary
 
-  s.required_ruby_version = '~> 2.5'
+  s.required_ruby_version = '>= 2.5'
 
   s.authors     = ['Roman Smirnov', 'Brian Quinn', 'Allison Reilly']
   s.email       = '[email protected]'