Fix @Stylesheet annotation for compatibility with Vaadin 16

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/orgchart/OrgChart.java

@@ -51,7 +51,7 @@
 @NpmPackage(value = "jquery", version = "3.4.1")
 @JsModule("jquery/dist/jquery.js")
 @JsModule("./orgchart/dist/js/jquery.orgchart.js")
-@StyleSheet("./jquery.orgchart.min.css")
+@StyleSheet("context://frontend/jquery.orgchart.min.css")
 @Tag("fc-orgchart")
 @JsModule("./fc-orgchart.js")
 public class OrgChart extends Div {