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-http/src/main/java/org/springframework/integration/http/inbound/IntegrationRequestMappingHandlerMapping.java

@@ -82,12 +82,12 @@ public final class IntegrationRequestMappingHandlerMapping extends RequestMappin
 	private final AtomicBoolean initialized = new AtomicBoolean();
 
 	@Override
-	protected final boolean isHandler(Class<?> beanType) {
+	protected boolean isHandler(Class<?> beanType) {
 		return HttpRequestHandlingEndpointSupport.class.isAssignableFrom(beanType);
 	}
 
 	@Override
-	protected final HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
+	protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
 		if (handler instanceof HandlerMethod) {
 			HandlerMethod handlerMethod = (HandlerMethod) handler;
 			Object bean = handlerMethod.getBean();
@@ -109,7 +109,7 @@ public final class IntegrationRequestMappingHandlerMapping extends RequestMappin
 	}
 
 	@Override
-	protected final void detectHandlerMethods(Object handler) {
+	protected void detectHandlerMethods(Object handler) {
 		if (handler instanceof String) {
 			handler = this.getApplicationContext().getBean((String) handler);
 		}