Update to Tomcat 9

Spring recently updated to require Servlet 4, so update to Tomcat 9.

Closes gh-6

servlet/java-configuration/authentication/preauth/server/tomcat-users.xml

@@ -0,0 +1,19 @@
+<!--
+  ~ Copyright 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.
+  ~ You may obtain a copy of the License at
+  ~
+  ~      https://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+
+<tomcat-users>
+    <user username="user" password="password" roles="ROLE_USER" />
+</tomcat-users>