Update to checkout@v6

Author: Aftersol

.github/workflows/cmake-multi-platform.yml

@@ -53,7 +53,7 @@ jobs:
             c_compiler: cl
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
 
     - name: Set reusable strings
       # Turn repeated input strings (such as the build output directory) into step outputs. These step outputs can be used throughout the workflow file.