Update incorrect Core version. (#5949)

Author: ryanwilson

Firebase.podspec

@@ -36,7 +36,7 @@ Simplify your app development, grow your user base, and monetize more effectivel
   end
 
   s.subspec 'CoreOnly' do |ss|
-    ss.dependency 'FirebaseCore', '6.8.0'
+    ss.dependency 'FirebaseCore', '6.8.1'
     ss.source_files = 'CoreOnly/Sources/Firebase.h'
     ss.preserve_paths = 'CoreOnly/Sources/module.modulemap'
     if ENV['FIREBASE_POD_REPO_FOR_DEV_POD'] then