Previously, UrlPathHelper's full path optimization was enabled when
there was a dispatcher servlet mapped to /. The UrlPathHelper is used
across Spring MVC and if there are multiple dispatcher servlets they
all share the sample UrlPathHelper. This meant that any additional
dispatcher servlets mapping to locations other than / would not be able
to map requests correctly as the UrlPathHelper would use the full path,
ignoring the url mapping of the dispatcher servlet.

This commit updates the MVC auto-configuration so that use of the full
path is only enabled if there's a single dispatcher servlet
registration.

Fixes gh-22682