Remove @Configuration meta-annotation from @Enable annotations

[vpavic]

Currently, all Spring Session's @Enable annotations are meta-annotated with @Configuration. While convenient, this is not consistent with the rest of the Spring projects and most notably Spring Framework's @Enable annotations.

Additionally, the introduction of support for @Configuration(proxyBeanMethods=false) in Spring Framework provides a compelling reason to remove @Configuration meta-annotation from Spring Session's @Enable annotations and allow users to opt into their preferred configuration mode.