CI: bump checkout action to v7

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

Author: vstehle

.github/workflows/main.yaml

@@ -20,7 +20,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@v7
         # Make sure we fetch the tags for proper version identification.
         with:
           fetch-depth: 0