Spring Simplify MockHttpServletRequestBuilders hierarchy

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 https://github.com/spring-projects/spring-framework/commit/265299c790fcec55d2f3de12b91e43e2361523e7