Improve doc for 1.10.x support (#13720)
Improve doc for 1.10.x support (#13720)

- Remove duplicate information
- Use specific time (6 months) instead of "limited time"
lyft/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,apache/airflow,lyft/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,mistercrunch/airflow,bolkedebruin/airflow,apache/airflow,bolkedebruin/airflow,apache/incubator-airflow,apache/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,apache/incubator-airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,nathanielvarona/airflow,sekikn/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,apache/airflow,bolkedebruin/airflow,apache/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,dhuang/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,apache/incubator-airflow,apache/incubator-airflow,dhuang/incubator-airflow