chore: group Bigtable dependency updates (#8137)

Grouping Bigtable updates to reduce clutter and ensure consistency. This covers 4 currently open updates, all on 2.8.1.

Author: kweinmeister

renovate.json

@@ -16,6 +16,12 @@
       ],
       "groupName": "AppEngine packages"
     },
+    {
+      "packagePatterns": [
+        "^com.google.cloud.bigtable:"
+      ],
+      "groupName": "Bigtable packages"
+    },    
     {
       "matchPackagePrefixes": [ "com.google" ],
       "allowedVersions": "!/.+-sp\\.[0-9]+$/"