Link to releases repo for upgrade issues (#43295)

Summary:
Changelog: [Internal]

We're moving upgrade issues to https://github.com/reactwg/react-native-releases/issues/new/choose

Pull Request resolved: https://github.com/facebook/react-native/pull/43295

Reviewed By: cortinico, cipolleschi

Differential Revision: D54442540

Pulled By: lunaleaps

fbshipit-source-id: a079935a290f56d2932c5fc588e96a68e1c13f9a