**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

airflow/configuration.py
return old.search(current_value) is not None
    def _update_env_var(self, section, name, new_value):
        
        
        env_var = self._env_var_name(section, name)
        os.environ.pop(env_var, None)
        if not self.has_section(section):
            self.add_section(section)
        self.set(section, name, new_value)
    @staticmethod
    def _create_future_warning(name, section, current_value, new_value, version):
        warnings.warn(
            'The {name} setting in [{section}] has the old default value '
            'of {current_value!r}. This value has been changed to {new_value!r} in the '
            'running config, but please update your config before Apache '
            'Airflow {version}.'.format(
                name=name, section=section, current_value=current_value, new_value=new_value, version=version
            ),
            FutureWarning,
        )
    ENV_VAR_PREFIX = 'AIRFLOW__'
    def _env_var_name(self, section: str, key: str) -> str: