Deprecate EhCache2 support

Since EhCache 3 is fully JSR-107 compliant, we should remove EhCache2 support and provide JCache implementations

Closes gh-10362

core/src/main/java/org/springframework/security/core/userdetails/cache/EhCacheBasedUserCache.java

@@ -32,7 +32,9 @@ import org.springframework.util.Assert;
  * <A HREF="https://www.ehcache.org/">EHCACHE</a>.
  *
  * @author Ben Alex
+ * @deprecated since 5.6. In favor of JCache based implementations
  */
+@Deprecated
 public class EhCacheBasedUserCache implements UserCache, InitializingBean {
 
 	private static final Log logger = LogFactory.getLog(EhCacheBasedUserCache.class);