build: upgrade Vaadin to 14.8.20

Author: javier-godoy

pom.xml

@@ -6,7 +6,7 @@
 	<version>2.9.3-SNAPSHOT</version>
 	<name>TwinColGrid add-on</name>
     <properties>
-        <vaadin.version>14.9.7</vaadin.version>
+        <vaadin.version>14.8.20</vaadin.version>
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>