Move to checkout@v6 to avoid the deprecated Node 20.

The v4 version of this action uses the deprecated Node 20, which
triggers several warnings in every CI job.

Author: hageboeck

.github/workflows/cling.yml

@@ -19,15 +19,15 @@ jobs:
           sudo apt-get install -y libedit-dev
 
       - name: Checkout LLVM
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           repository: root-project/llvm-project
           path: llvm
           ref: cling-latest
           fetch-depth: 1
 
       - name: Checkout Cling
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           repository: root-project/cling
           path: cling