chore(deps): update gcr.io/cloud-spanner-emulator/emulator docker tag to v1.5.51

Author: renovate-bot

.github/workflows/emulator-system-tests-spanner.yaml

@@ -18,7 +18,7 @@ jobs:
     runs-on: 'ubuntu-24.04'
     services:
       emulator:
-        image: gcr.io/cloud-spanner-emulator/emulator:1.5.48
+        image: gcr.io/cloud-spanner-emulator/emulator:1.5.51
         ports:
           - 9010:9010
           - 9020:9020