🤖 Update actions/checkout action to v6 (#210)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/images.yml

@@ -18,7 +18,7 @@ jobs:
       packages: write
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
     - uses: docker/login-action@v3

.github/workflows/verify.yml

@@ -23,7 +23,7 @@ jobs:
         working-directory: go/src/github.com/stackitcloud/gardener-extension-shoot-flux
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         path: go/src/github.com/stackitcloud/gardener-extension-shoot-flux
     - uses: actions/setup-go@v6