This changes `InMemoryOAuth2AuthorizedClientService.loadAuthorizedClient`
(and its reactive counterpart) to always return `OAuth2AuthorizedClient`
instances containing the current `ClientRegistration` as obtained from
the `ClientRegistrationRepository`.
Before this change, the first `ClientRegistration` instance was cached,
with the effect that any changes made in the `ClientRegistrationRepository`
(such as a new client secret) would not have taken effect.
Closes gh-15511
73f3f75712