Configure allowed origins for GraphQL WebSocket support

As of spring-projects/spring-graphql#1465, Spring for GraphQL also
supports applying allowed origins to the websocket endpoint.
As it's considered a bug of omission in that project, this commit
adapts the Spring Boot auto-configuration to leverage this change.

There is no need for new configuration properties, as
`spring.graphql.cors.*` origin-related keys apply to all supported
transports.

Fixes gh-50391