Make Grape API defintion compatible with Ruby 3

As suggested by the gem author:
https://github.com/LeFnord/grape-entity/blob/master/UPGRADING.md

lib/api/entities/blob.rb

@@ -10,7 +10,7 @@ module API
       # in the future we can only return the filename here without the leading
       # directory path.
       # https://gitlab.com/gitlab-org/gitlab/issues/34521
-      expose :filename, &:path
+      expose :path, as: :filename
       expose :id
       expose :ref
       expose :startline