Prior to this change, the reference docs would point to "spring-webflux"
as the required additional testing dependency to get
`WebGraphQlTester` support in Spring Boot tests.
While this is enough for `WebEnvironment.MOCK` tests, we need an actual
HTTP client for `WebEnvironment.RANDOM_PORT` tests. This commit amends
this part of the documentation to recommend the
"spring-boot-starter-webflux" dependency in all cases.
Fixes gh-29250
b17412eb38