Remove 'spring.http.client' deprecation and change 'spring.http.reactiveclient.settings' to 'spring.http.reactiveclient'

[philwebb]

In 3.5.0 we deprecated spring.http.client in favor of spring.http.client.settings. The main aim was to free up spring.http.client.<group> for interface clients in 4.0. Ultimately, we've decided to go with spring.http.client.service.group.<group> in 4.0 so the settings change is unnecessary and arguably makes things harder to read.

Although we've cut RC1, I think we should remove the .settings suffix from spring.http.client.settings and spring.http.reactiveclient.settings before we GA.