**Description**

It would be nice if the weight_rule default value could be managed by a global config

suggested config:
```
# Weighting method used for the effective total priority weight of the task. 
# Options are: { downstream | upstream | absolute } default is
default_weight_rule = downstream
```
**Use case / motivation**

In some pipeline, you really need to have absolute weight, and then you have to add a line in each task definition which is annoying

tests/core/test_configuration.py
test_conf = AirflowConfigParser(default_config='')
        
        
        test_conf.deprecated_values = {
            'core': {'hostname_callable': (re.compile(r':'), r'.', '2.1')},
        }
        test_conf.read_dict({'core': {'hostname_callable': 'socket:getfqdn'}})
        with pytest.warns(FutureWarning):
            test_conf.validate()
            assert test_conf.get('core', 'hostname_callable') == 'socket.getfqdn'
    @pytest.mark.parametrize(
        "conf_dict",
        [
            {},  
            {'core': {'hostname_callable': 'socket:getfqdn'}},
        ],
    )
    def test_deprecated_values_from_environ(self, conf_dict):