Update CI Configuration for IRuby (#344)

- Deleted Ruby 2.5 and 2.6 from the CI matrix
- Removed IRB installation logic for older Ruby versions
- Added ffi 1.6 installation for Ruby 2.7
- Update checkout action to v4 in CI workflow

Author: kojix2

.github/workflows/ci.yml

@@ -26,12 +26,10 @@ jobs:
           - "3.1"
           - "3.0"
           - "2.7"
-          - "2.6"
-          - "2.5"
           - debug
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 1
 
@@ -41,15 +39,13 @@ jobs:
 
       - run: rake build
 
-      - name: Install irb for old Ruby
+      - name: Install ffi 1.6 for old Ruby
         if: |
-          matrix.ruby == '2.5' ||
-          matrix.ruby == '2.4' ||
-          matrix.ruby == '2.3'
+          matrix.ruby == '2.7'
         run: |
           cat <<GEMFILE > Gemfile.irb
           source 'https://rubygems.org'
-          gem 'irb'
+          gem 'ffi', '~> 1.6'
           GEMFILE
           BUNDLE_GEMFILE=Gemfile.irb bundle install --jobs 4 --retry 3
 
@@ -87,13 +83,13 @@ jobs:
     runs-on: windows-latest
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 1
 
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.0"
+          ruby-version: "ruby"
 
       - run: rake build
 
@@ -108,13 +104,13 @@ jobs:
     runs-on: macos-latest
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 1
 
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.0"
+          ruby-version: "ruby"
 
       - run: rake build