docs: fix CORS config paths to use server.webflux prefix

[tfahim007]

docs: fix CORS configuration paths in webflux documentation

Update the CORS configuration examples to use the correct property
paths under spring.cloud.gateway.server.webflux instead of the
incorrect spring.cloud.gateway paths.

Changes:

• Global CORS configuration now uses
  spring.cloud.gateway.server.webflux.globalcors
• Property reference updated to
  spring.cloud.gateway.server.webflux.globalcors.add-to-simple-url-handler-mapping
• Route CORS configuration example updated to use
  spring.cloud.gateway.server.webflux.routes for consistency

This aligns the documentation with the actual implementation where
GatewayProperties.PREFIX is "spring.cloud.gateway.server.webflux"
and GlobalCorsProperties uses GatewayProperties.PREFIX + ".globalcors".

[tfahim007]

@spencergibb This PR corrects the CORS configuration property paths in the WebFlux documentation, which currently do not align with the actual implementation. The existing examples have been misleading users by referencing spring.cloud.gateway.* instead of the correct spring.cloud.gateway.server.webflux.* prefix.

Kindly review this change, as it brings the documentation in line with GatewayProperties.PREFIX and GlobalCorsProperties, and should help prevent further user confusion.