Review PathMappings put(PathSpec, E resource) for replacement behavior

**Jetty version(s)**
Jetty 12

**Description**
As pointed out at https://github.com/eclipse/jetty.project/pull/8748#discussion_r1003823744 if a new mapping is introduced that would replace an existing PathSpec,
we should be returning the old resource that was replaced in the `put()` method.