ci: add ruby v4

Author: KazuCocoa

.github/workflows/unittest.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [3.1, 3.2, 3.3]
+        ruby: [3.1, 3.2, 3.3, 4.0]
 
     runs-on: ubuntu-latest
 
@@ -39,7 +39,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [3.1, 3.2, 3.3]
+        ruby: [3.1, 3.2, 3.3, 4.0]
 
     runs-on: windows-latest