Drop Ruby 2.5/2.6 support

Both ruby version are EoL.

We've another breaking change that I need to release,
voxpupuli#515. 2.7 is EoL as well,
but still widely used in the Puppet Ecosystem, so I think switching to
2.7 for now is fine.

.github/workflows/test.yml

@@ -19,10 +19,8 @@ jobs:
           - ruby: truffleruby
           - ruby: truffleruby+graalvm
           - ruby: jruby
-          - ruby: "2.5"
-            rake_task: "rubocop test"
-          - ruby: "2.6"
           - ruby: "2.7"
+            rake_task: "rubocop test"
           - ruby: "3.0"
           - ruby: "3.1"
             coverage: "yes"

json-schema.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |s|
   s.files = Dir['lib/**/*', 'resources/*.json']
   s.require_path = 'lib'
   s.extra_rdoc_files = ['README.md', 'LICENSE.md']
-  s.required_ruby_version = '>= 2.5'
+  s.required_ruby_version = '>= 2.7'
   s.license = 'MIT'
   s.required_rubygems_version = '>= 2.5'