Add deprecated "user" filter for posts

In the filterer refactor for ListPostsController, the filter key was changed to `author` for consistency with the AuthorFilterGambit used in discussions. This commit adds a deprecated `user` filter back in for a release to allow for a graceful transition

src/Filter/FilterServiceProvider.php

@@ -46,6 +46,7 @@ class FilterServiceProvider extends AbstractServiceProvider
                     PostFilter\IdFilter::class,
                     PostFilter\NumberFilter::class,
                     PostFilter\TypeFilter::class,
+                    PostFilter\UserFilter::class,
                 ],
             ];
         });