Add details around how long Airflow will maintain 1.10.x series ?

[kaxil]

Since we are close to Airflow 2, there have been questions from users around how long will Airflow 1.10.x series be maintained. This needs to be added to a user-facing docs, maybe in https://github.com/apache/airflow/blob/master/UPGRADING_TO_2.0.md

• How frequently will we release it?
• Will it only include security patches?

cc @XD-DENG @vikramkoka

[kaxil]

In one of the Dev calls (link) we agreed the following:

• Only critical fixes (fixes to bugs that takedown Production system) will be backported to 1.10.x core for six months after Airflow 2.0 is released.
• Backport providers will only be supported/released for three months after 2.0.0 released