Move spring.oauth2.* to security.oauth2.*

Unfortunately, we have no other choice to flip the ignoreUnknownFields
attribute of `SecurityProperties` has many different target are now set
for that namespace outside the class. See gh-3445 for a potential way
to improve that.

Closes gh-3327

spring-boot-samples/spring-boot-sample-secure-oauth2/src/main/resources/application.properties

@@ -1,8 +1,8 @@
 spring.datasource.platform=h2
-spring.oauth2.client.client-id=foo
-spring.oauth2.client.client-secret=bar
 
 security.user.name=greg
 security.user.password=turnquist
+security.oauth2.client.client-id=foo
+security.oauth2.client.client-secret=bar
 
 logging.level.org.springframework.security=DEBUG