Prior to this commit, the `HtmlUnitRequestBuilder` would "translate"
HtmlUnit web requests into Servlet requests using APIs that were not
clearly defined and meant for internal usage.
HtmlUnit 2.64.0 introduced a new `.getParameters()` API for collecting
`NameValuePair` parsed from the request URI or request body, depending
on the nature of the request. This arrangement is much more stable and
in line with HtmlUnit's expectations.
This commit uses this new API and makes HtmlUnit 2.64.0 a new minimum
requirement for using HtmlUnit integration with Spring Framework.
This also removes tests that were previously testing HtmlUnit's behavior
and using the API now marked as internal.
Closes gh-28240
