Update CI workflow to include Ruby 4.0 in testing matrix

Author: ydah

.github/workflows/main.yml

@@ -9,8 +9,16 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest]
-        ruby: ['3.1', '3.2', '3.3', '3.4', 'head']
+        os:
+          - ubuntu-latest
+          - macos-latest
+        ruby:
+          - '3.1'
+          - '3.2'
+          - '3.3'
+          - '3.4'
+          - '4.0'
+          - 'head'
     runs-on: ${{ matrix.os }}
     name: Ruby ${{ matrix.ruby }} / ${{ matrix.os }}
     steps: