Airflow issue cleanup

[Vikram Koka <vi...@astronomer.io>]

Hey Airflow devs,

I have been cleaning up the Airflow issues in Github, so that we have a
clean view of:
1. Issues which are blocking Airflow 2.0 beta1
2. Issues which should be in Airflow 2.0 beta2
3. Issues which should be in Airflow 2.0.0
4. Issues beyond Airflow 2.0, which I have been assigning to Airflow 2.1

Specifically, I have primarily been changing milestone assignments and
adding labels where appropriate.

Admittedly, this is still very incomplete, but wanted you to be aware of
why you are receiving alerts from Github about changes to issues with
respect to milestones and so on.

Any and all feedback is most welcome! For those of you who have already
responded, thank you so much!

Best regards,

Vikram