tomcat service: fix webapps default option (#40657)

The old package tomcat.webapps does not exist

nixos/modules/services/web-servers/tomcat.nix

···
 
       webapps = mkOption {
         type = types.listOf types.package;
-        default = [ tomcat.webapps ];
-        defaultText = "[ tomcat.webapps ]";
+        default = [ tomcat85.webapps ];
+        defaultText = "[ tomcat85.webapps ]";
         description = "List containing WAR files or directories with WAR files which are web applications to be deployed on Tomcat";
       };