Drop Ruby 2.5 and 2.6 support

They're EOL. So are 2.7 and 3.0, but I want to leave a 2.x just in case
someone is lagging.

Related: #178

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        ruby: [2.5, 2.6, 2.7, 3.0, 3.1, 3.2, 3.3, head, jruby, jruby-head, truffleruby, truffleruby-head]
+        ruby: [2.7, 3.0, 3.1, 3.2, 3.3, head, jruby, jruby-head, truffleruby, truffleruby-head]
         exclude:
           - os: ubuntu-latest
             ruby: head

json_schemer.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.required_ruby_version = '>= 2.5'
+  spec.required_ruby_version = '>= 2.7'
 
   spec.add_development_dependency "base64"
   spec.add_development_dependency "bundler", "~> 2.0"