Change the number of days to kill stales from 3 to 7 (#21015)

Summary:
Basically, saw this commit this morning and had a chance to discuss it with matthargett and AndrewJack and overall I agree with their criticism about 3 days being too little time for people to react to it.
Pull Request resolved: https://github.com/facebook/react-native/pull/21015

Differential Revision: D9721393

Pulled By: hramos

fbshipit-source-id: 524c0b87987a8b76fa8d5e223fb5ed5a56a8c648