Previously, when a user had declared a custom MultipartResolver bean
that is a CommonsMultipartResolver, part resolution would fail. The
failure was occurring as the servlet container was consuming the parts
before CommonsMultipartResolver had a chance to read them. This was
happening because a MultipartConfigElement was being auto-configured.

This commit updates the multipart auto-configuration so that a
MultipartConfigElement is not auto-configured when there is a
CommonsMultipartResolver bean in the context.

Closes gh-7735