feat(ios, sdk): bump firebase-ios-sdk to 8.15.0

Not new features for react-native-firebase but they went minor version
so will make it a minor version here

Author: mikehardy

packages/app/package.json

@@ -65,7 +65,7 @@
   },
   "sdkVersions": {
     "ios": {
-      "firebase": "8.14.0"
+      "firebase": "8.15.0"
     },
     "android": {
       "minSdk": 19,