Sync DB Migrations in Master with 2.0.1 (#14155)

`449b4072c2da_increase_size_of_connection_extra_field_.py` does not exist in 2.0.1

so we need to update down_migration

```
INFO  [alembic.runtime.migration] Running upgrade 2c6edca13270 -> 61ec73d9401f, Add description field to connection
INFO  [alembic.runtime.migration] Running upgrade 61ec73d9401f -> 64a7d6477aae, fix description field in connection to be text
INFO  [alembic.runtime.migration] Running upgrade 64a7d6477aae -> e959f08ac86c, Change field in DagCode to MEDIUMTEXT for MySql
INFO  [alembic.runtime.migration] Running upgrade e959f08ac86c -> 82b7c48c147f, Remove can_read permission on config resource for User and Viewer role
[2021-02-09 19:17:31,307] {migration.py:555} INFO - Running upgrade 82b7c48c147f -> 449b4072c2da, Increase size of connection.extra field to handle multiple RSA keys
```

(cherry picked from commit e7f176d)

airflow/migrations/versions/449b4072c2da_increase_size_of_connection_extra_field_.py

@@ -19,7 +19,7 @@
 """Increase size of connection.extra field to handle multiple RSA keys
 
 Revision ID: 449b4072c2da
-Revises: e959f08ac86c
+Revises: 82b7c48c147f
 Create Date: 2020-03-16 19:02:55.337710
 
 """
@@ -29,7 +29,7 @@
 
 # revision identifiers, used by Alembic.
 revision = '449b4072c2da'
-down_revision = 'e959f08ac86c'
+down_revision = '82b7c48c147f'
 branch_labels = None
 depends_on = None