Replace probot stale checks with oppiabot

[U8NWXD]

It looks like we currently use probot running on Heroku to check for stale PRs. We should implement this in Oppiabot directly instead.

Possibly relevant:

• A fork of probot that might be running our stale checks: https://github.com/oppia/oppiabot-stale
• An existing utility to check for stale PRs:

  oppiabot/lib/utils.js

  Line 69 in 3fba86a

  const checkPrIsStale = async (pullRequest, context) => {

• We have oppia/oppiabot-stale as a dependency:

  oppiabot/package.json

  Line 28 in 3fba86a

  "probot-stale": "github:oppia/oppiabot-stale"