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

Differential Revision: D9721393

Pulled By: hramos

fbshipit-source-id: 524c0b87987a8b76fa8d5e223fb5ed5a56a8c648

.github/stale.yml

@@ -2,7 +2,7 @@
 # Number of days of inactivity before an issue becomes stale
 daysUntilStale: 90
 # Number of days of inactivity before a stale issue is closed
-daysUntilClose: 3
+daysUntilClose: 7
 # Issues with these labels will never be considered stale
 exemptLabels:
   - Good first issue