ci: align integration workflow to actions/checkout@v6

Matches the version Dependabot just bumped ci.yml to, so the new workflow
doesn't immediately trigger another bump PR.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rustyconover

.github/workflows/integration.yml

@@ -53,10 +53,10 @@ jobs:
     name: integration (${{ matrix.lane }})
     steps:
       - name: Checkout vgi-python
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Checkout pinned vgi test suite
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           repository: Query-farm/vgi
           ref: ${{ env.VGI_REF }}