chore(deps): update dependency @google-cloud/spanner to v6 (GoogleCloudPlatform#151)

Author: renovate-bot

package.json

@@ -36,7 +36,7 @@
   },
   "devDependencies": {
     "@grpc/proto-loader": "^0.6.2",
-    "@google-cloud/spanner": "^5.12.0",
+    "@google-cloud/spanner": "^6.0.0",
     "c8": "^7.7.2",
     "google-auth-library": "^8.0.0",
     "google-gax": "^2.14.1",