Deprecate passing most Legend arguments positionally (matplotlib#23166)

There are so many of them, it really doesn't make sense to do
positionally.

Author: QuLogic

doc/api/next_api_changes/deprecations/23166-ES.rst

@@ -0,0 +1,5 @@
+``Legend`` constructor
+~~~~~~~~~~~~~~~~~~~~~~
+
+All arguments to `.legend.Legend` other than *parent*, *handles*, and *labels*
+will become keyword-only in a future version.

lib/matplotlib/legend.py

@@ -294,6 +294,7 @@ class Legend(Artist):
     def __str__(self):
         return "Legend"
 
+    @_api.make_keyword_only("3.6", "loc")
     @_docstring.dedent_interpd
     def __init__(
         self, parent, handles, labels,