Sanitize caption used in compatibility Grid header (#12018)

* Sanitize caption used in compatibility Grid header

Cherry pick of #11644

compatibility-server/src/main/java/com/vaadin/v7/ui/Grid.java

 import java.util.logging.Level;
 import java.util.logging.Logger;
 
+import org.jsoup.Jsoup;
 import org.jsoup.nodes.Attributes;
 import org.jsoup.nodes.Element;
 import org.jsoup.select.Elements;
             if (caption == null) {
                 caption = ""; // Render null as empty
             }
+            caption = Jsoup.parse(caption).text();
             state.headerCaption = caption;
 
             HeaderRow row = grid.getHeader().getDefaultRow();