chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/docker.yml

@@ -21,7 +21,7 @@ jobs:
       SQLX_OFFLINE: true
     steps:
       - name: Checkout sources
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Install OpenSSL build deps
         if: runner.os == 'Linux'

.github/workflows/rust.yml

@@ -30,7 +30,7 @@ jobs:
             artifact_name: stacker-macos-aarch64
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Verify .sqlx cache exists
         run: |
           ls -lh .sqlx/ || echo ".sqlx directory not found"