Fix MySQL database character set instruction (apache#17603)

The currently instructed character set `utf8mb4 COLLATE utf8mb4_unicode_ci;` does not work on mysql 8. 
When I do: `airflow db init` the following error occurs:
`
sqlalchemy.exc.OperationalError: (MySQLdb._exceptions.OperationalError) (1071, 'Specified key was too long; max key length is 3072 bytes')
`
Changing to this character set: `utf8 COLLATE utf8_general_ci;` solved the problem

docs/apache-airflow/howto/set-up-database.rst

@@ -154,7 +154,7 @@ In the example below, a database ``airflow_db`` and user  with username ``airflo
 
 .. code-block:: sql
 
-   CREATE DATABASE airflow_db CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+   CREATE DATABASE airflow_db CHARACTER SET utf8 COLLATE utf8_general_ci;
    CREATE USER 'airflow_user' IDENTIFIED BY 'airflow_pass';
    GRANT ALL PRIVILEGES ON airflow_db.* TO 'airflow_user';