add security features + java property support

- set `ALLOW_PLANTUML_INCLUDE` only once and decentralized inside the `DiagramResponse` class and call this init method after initializing the server
- set `PLANTUML_SECURITY_PROFILE` to `INTERNET` by default (BREAKING CHANGES)
- add possibility to set PlantUML system properties over a file with `PLANTUML_PROPERTY_FILE`
- adjust documentation
- add "Breaking changes" hint to README

src/main/java/net/sourceforge/plantuml/servlet/ProxyServlet.java

@@ -41,7 +41,6 @@ import jakarta.servlet.http.HttpServletResponse;
 
 import net.sourceforge.plantuml.BlockUml;
 import net.sourceforge.plantuml.FileFormat;
-import net.sourceforge.plantuml.OptionFlags;
 import net.sourceforge.plantuml.SourceStringReader;
 import net.sourceforge.plantuml.core.Diagram;
 import net.sourceforge.plantuml.core.UmlSource;
@@ -54,13 +53,6 @@ import net.sourceforge.plantuml.core.UmlSource;
 @SuppressWarnings("SERIAL")
 public class ProxyServlet extends HttpServlet {
 
-    static {
-        OptionFlags.ALLOW_INCLUDE = false;
-        if ("true".equalsIgnoreCase(System.getenv("ALLOW_PLANTUML_INCLUDE"))) {
-            OptionFlags.ALLOW_INCLUDE = true;
-        }
-    }
-
     public static boolean forbiddenURL(String full) {
         if (full == null) {
             return true;