Enable RedundantModifier checkstyle, add fixer

Add `fixModifiers.gradle` to run after `check` task.

The `RedundantModifierChecker` doesn't catch all errors at once.
We need run `check -> fixModifiers` pair several times to reach `SUCCESSFUL`.

The `fixThis` has been ported from the SA, but without applying.

* Polishing for `fixModifiers.gradle`
* Fix all redundant modifier with the `fixModifiers.gradle`

NOTE: Since all these task modify files on Windows we have to run them with the `-Dfile.encoding=UTF-8`.
Otherwise they are read and write with the Windows default `cp1251` making them incompatible with Unix system.

spring-integration-websocket/src/test/java/org/springframework/integration/websocket/client/StompIntegrationTests.java

@@ -435,7 +435,7 @@ public class StompIntegrationTests {
 
 	@MessagingGateway
 	@Controller
-	static interface WebSocketGateway {
+	interface WebSocketGateway {
 
 		@MessageMapping("/greeting")
 		@SendToUser("/queue/answer")