[Combine] Update FirebaseCore pod dep to allow patch updates (#14115)

Author: andrewheard

FirebaseCombineSwift.podspec

@@ -51,7 +51,7 @@ for internal testing only. It should not be published.
   s.osx.framework = 'AppKit'
   s.tvos.framework = 'UIKit'
 
-  s.dependency 'FirebaseCore', '11.6.0'
+  s.dependency 'FirebaseCore', '~> 11.6.0'
   s.dependency 'FirebaseAuth', '~> 11.0'
   s.dependency 'FirebaseFunctions', '~> 11.0'
   s.dependency 'FirebaseFirestore', '~> 11.0'