Preserve X-Forwarded-Prefix handling when upgrading to Spring Boot 4.2

Spring Boot 4.2 ignores the X-Forwarded-Prefix header unless use-forwarded-prefix is enabled for the forward headers strategy in use. Enable it in Spring configuration files that set server.forward-headers-strategy to framework in Spring MVC and WebFlux applications, or to native in WebFlux applications, to preserve the behavior before Spring Boot 4.2. Applications that rely on a cloud platform to select the native strategy are not changed. Explicit configuration is retained.