Reformat 2.4.0 release notes (#26247)
Reformat 2.4.0 release notes (#26247)

This makes them proper sections and reorders a couple of them as well.
cfei18/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,apache/airflow,nathanielvarona/airflow,apache/airflow,cfei18/incubator-airflow,apache/airflow,apache/airflow,cfei18/incubator-airflow,apache/airflow,cfei18/incubator-airflow,apache/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,nathanielvarona/airflow