Actions: Update checkout to v6

Author: simmsa

.github/workflows/main.yml

@@ -51,7 +51,7 @@ jobs:
       should-run-hindcast: ${{ steps.hindcast-logic.outputs.should-run-hindcast }}
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Check for changes in wave/io/hindcast
         id: changes