Update main source and target JDK compatibility
Update main source and target JDK compatibility

Update source and target compatibility from 1.5 to 1.6, meaning that
@Override is now allowed on implementations of interface methods within
main classes.

Issue: SPR-10130

spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework