Drop to support Ruby 2.4

Author: hsbt

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     name: build (${{ matrix.ruby }} / ${{ matrix.os }})
     strategy:
       matrix:
-        ruby: [ '3.0', 2.7, 2.6, 2.5, 2.4, head ]
+        ruby: [ '3.0', 2.7, 2.6, 2.5, head ]
         os: [ ubuntu-latest, macos-latest ]
     runs-on: ${{ matrix.os }}
     steps:

mutex_m.gemspec

@@ -20,6 +20,7 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
+  spec.required_ruby_version = '>= 2.5'
 
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"