The migration added in #10873 doesn't work on Mysql8 -- it is too long for a text column with utf8 collation.

```
Running upgrade 2c6edca13270 -> 61ec73d9401f, Add description field to connection
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1277, in _execute_context
    cursor, statement, parameters, context
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/engine/default.py", line 593, in do_execute
    cursor.execute(statement, parameters)
  File "/usr/local/lib/python3.6/site-packages/MySQLdb/cursors.py", line 255, in execute
    self.errorhandler(self, exc, value)
  File "/usr/local/lib/python3.6/site-packages/MySQLdb/connections.py", line 50, in defaulterrorhandler
    raise errorvalue
  File "/usr/local/lib/python3.6/site-packages/MySQLdb/cursors.py", line 252, in execute
    res = self._query(query)
  File "/usr/local/lib/python3.6/site-packages/MySQLdb/cursors.py", line 378, in _query
    db.query(q)
  File "/usr/local/lib/python3.6/site-packages/MySQLdb/connections.py", line 280, in query
    _mysql.connection.query(self, query)
_mysql_exceptions.OperationalError: (1118, 'Row size too large. The maximum row size for the used table type, not counting BLOBs, is 65535. This includes storage overhead, check the manual. You have to change some columns to TEXT or BLOBs')
```

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airflow/models/connection.py
"""Return connection in URI format"""
        uri = '{}://'.format(str(self.conn_type).lower().replace('_', '-'))
        authority_block = ''
        if self.login is not None:
            authority_block += quote(self.login, safe='')
        if self.password is not None:
            authority_block += ':' + quote(self.password, safe='')
        if authority_block > '':
            authority_block += '@'
            uri += authority_block
        host_block = ''
        if self.host:
            host_block += quote(self.host, safe='')
        if self.port:
            if host_block > '':
                host_block += f':{self.port}'
            else:
                host_block += f'@:{self.port}'
        if self.schema:
            host_block += '/{}'.format(quote(self.schema, safe=''))
        uri += host_block
        if self.extra_dejson:
            uri += '?{}'.format(urlencode(self.extra_dejson))
        return uri
    def get_password(self) -> Optional[str]: