Add whitespace after copyright header

Add an additional lines after the copyright header and before the
`package` declaration. This aligns with the style used by Spring
Framework.

Issue gh-8945

rsocket/src/main/java/org/springframework/security/rsocket/util/matcher/PayloadExchangeMatcher.java

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.security.rsocket.util.matcher;
 
 import java.util.Collections;