This commit adds the checkNotModified method to ServerRequest in both
WebFlux.fn and WebMvc.fn. Unlike other checkNotModified methods found
in the framework, this method does not return a boolean, but rather
a response wrapped in a Mono/Optional. If the resource has
not been changed, the not-modified response can be returned directly;
if the resource has changed, the user can create a corresponding
response using switchIfEmpty/orElse(Get).
Closes gh-24173
0dc1c7eb8b