Add an `DispatcherServletPath` interface which provides a much more
consistent way to discover the path of the main dispatcher servet.

Prior to this commit, auto-configurations would often make use of the
`ServerProperties` class to discover the dispatcher servlet path. This
mechanism isn't very explicit and also makes it hard for us to relocate
that property in Spring Boot 2.1.

This commit also reverts most of fddc9e9c since it is now clear that
the supporting multiple dispatcher servlet paths will be much more
involved that we originally anticipated.

Closes gh-13834