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-web-secure-github/src/main/resources/application.yml

@@ -1,4 +1,4 @@
-spring:
+security:
   oauth2:
     client:
       clientId: bd1c0a783ccdd1c9b9e4