[BE] Remove deprecated github gql param and disable inconsistent test (#107385)
Two fixes:
- Stop querying `pushDate`, which [has been deprecated ](https://docs.github.com/en/graphql/reference/objects) and now always returns null
- Disables the test `test_merge_ghstack_into` which was recently added in https://github.com/pytorch/pytorch/pull/105251. This test used the results of another person's ghstack PR for testing, but as the dev submitted chunks of their PR this test's assumptions have been broken. cc @izaitsevfb for a long term fix here
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107385
Approved by: https://github.com/clee2000
