The compare to empty string checker (``pylint.extensions.emptystring``) and the compare to
zero checker (``pylint.extensions.compare-to-zero``) have been removed and their checks are
now part of the implicit booleaness checker:

``compare-to-zero`` was renamed ``use-implicit-booleaness-not-comparison-to-zero`` and
``compare-to-empty-string`` was renamed ``use-implicit-booleaness-not-comparison-to-string``
and they now need to be enabled explicitly.

The `pylint.extensions.emptystring`` and ``pylint.extensions.compare-to-zero`` extensions
no longer exists and needs to be removed from the ``load-plugins`` option.

Messages related to implicit booleaness were made more explicit and actionable.

This permits to make their likeness explicit and will provide better performance as they share most of their
conditions to be raised.

Closes #6871