Restore Servlet 3.1 compatibility

fixes codecentric#1387

Author: joshiste

spring-boot-admin-server-ui/src/main/java/de/codecentric/boot/admin/server/ui/web/servlet/HomepageForwardingFilter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2019 the original author or authors.
+ * Copyright 2014-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,6 +21,7 @@
 
 import javax.servlet.Filter;
 import javax.servlet.FilterChain;
+import javax.servlet.FilterConfig;
 import javax.servlet.ServletException;
 import javax.servlet.ServletRequest;
 import javax.servlet.ServletResponse;
@@ -42,7 +43,6 @@
  * history mode</a>
  */
 public class HomepageForwardingFilter implements Filter {
-
 private static final Logger log = LoggerFactory.getLogger(HomepageForwardingFilter.class);
 
 private final String homepage;
@@ -72,4 +72,11 @@ public void doFilter(ServletRequest request, ServletResponse response, FilterCha
 chain.doFilter(request, response);
 }
 
+@Override
+public void init(FilterConfig filterConfig) {
+}
+
+@Override
+public void destroy() {
+}
 }