Drop to support Ruby 2.4 because the latest version of core_assertions only support Ruby 2.5+

Author: hsbt

.github/workflows/libyaml.yml

@@ -7,7 +7,7 @@ jobs:
     uses: ruby/actions/.github/workflows/ruby_versions.yml@master
     with:
       engine: cruby
-      min_version: 2.4
+      min_version: 2.5
 
   build:
     needs: ruby-versions

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     uses: ruby/actions/.github/workflows/ruby_versions.yml@master
     with:
       engine: cruby-jruby
-      min_version: 2.4
+      min_version: 2.5
 
   test:
     needs: ruby-versions

psych.gemspec

@@ -42,7 +42,7 @@ DESCRIPTION
   s.rdoc_options = ["--main", "README.md"]
   s.extra_rdoc_files = ["README.md"]
 
-  s.required_ruby_version = Gem::Requirement.new(">= 2.4.0")
+  s.required_ruby_version = Gem::Requirement.new(">= 2.5.0")
   s.required_rubygems_version = Gem::Requirement.new(">= 0")
 
   if RUBY_ENGINE == 'jruby'