Simplify MockHttpServletRequestBuilders hierarchy

[snicoll]

In Spring Framework 6.2.x we introduced a hierarchy for MockHttpServletRequestBuilder and MockMultipartHttpServletRequestBuilder to add support for AssertJ. Unfortunately, this broke source and binary compatibility.

This has been restored in #33229 but we should take the opportunity of a major release to simplify the code.

This task is mostly about reverting 265299c