Prior to this commit, a change in `HttpStatus.toString` since SPR-16898
prevented the default WebFlux `ErrorWebExceptionHandler` to render
template views for exact HTTP status (e.g. "404.html").
This issue does not affect the resolution of series, like "4xx.html".

This commit fixes `DefaultErrorWebExceptionHandler` to use
`HttpStatus.value()` when attempting to resolve error views.

Closes gh-15083