Update to checkout action v6

Author: travkin79

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
 
     steps:
     - name: Check out
-      uses: actions/checkout@v4
+      uses: actions/checkout@v6
 
     - name: Set up JDK 21
       uses: actions/setup-java@v5 # see https://github.com/actions/setup-java
@@ -60,7 +60,7 @@ jobs:
 
     steps:
     - name: Check out
-      uses: actions/checkout@v4
+      uses: actions/checkout@v6
 
     - name: Set up JDK 21
       uses: actions/setup-java@v5