Increase n days before issue considered stale

Summary:
This repository has Probot's [Stale workflow](https://github.com/probot/stale) enabled. It is configured via the `.github/stale.yml` file.

In this PR, we increase number of days before an issue becomes stale to ~six months, and increase days until the same issue is closed after inactivity to ~2 months.

It also limits the stale bot to issues only.
Closes https://github.com/facebook/react-native/pull/18108

Differential Revision: D7105801

Pulled By: hramos

fbshipit-source-id: 23efd41f88c03cb6c0be3820a3b3ac67b3e4a6fe