[1.7.x] Fixed #23615 -- Validate that a Model instance's "check" attribute is a method.

The "check" name is a reserved word used by Django's check framework,
and cannot be redefined as something else other than a method, or the check
framework will raise an error.

This change amends the django.core.checks.model_check.check_all_models()
function, so that it verifies that a model instance's attribute "check"
is actually a method. This new check is assigned the id "models.E020".

Conflicts:
	docs/ref/checks.txt

Backport of a5c77417a6 from master

docs/ref/checks.txt

@@ -56,6 +56,7 @@ Models
 * **models.E015**: ``ordering`` refers to the non-existent field
   ``<field name>``.
 * **models.E017**: Proxy model ``<model>`` contains model fields.
+* **models.E020**: The ``<model>.check()`` class method is currently overridden.
 
 Fields
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