Use explicit JDK versions in aspects.gradle

Previously aspects.gradle used the Gradle conventions for the source
and target compatibility. This means that unless the conventions were
updated the current JDK would be used for both source and target
compatibilty. Since an update to build.gradle changed to configure the
compileJava and compileTestJava tasks explicitly spring-aspects has
been compiled with JDK 7 compatibility.

This commit explicitly uses the source and target compatibility from
spring-core to ensure that aspects.gradle is kept up to date.

Issue: SPR-10161