chore(app, ios): specific precise version of ios sdk

Tests are performed with specific versions, so a minor version range even
is not truly correct

Side effect is that this variable is now useful for git references without
parsing

Override testing was performed, if you override the FirebaseSDKVersion it still
performs correctly

Author: mikehardy

packages/app/package.json

@@ -62,7 +62,7 @@
   },
   "sdkVersions": {
     "ios": {
-      "firebase": "~> 7.6.0"
+      "firebase": "7.6.0"
     },
     "android": {
       "minSdk": 16,