build: update Vaadin version to 14.6.8

Vaadin 14.6 is required in order to use the DefaultErrorHandler annotation.

Author: javier-godoy

pom.xml

@@ -37,7 +37,7 @@
 	</licenses>
 
 	<properties>
-        <vaadin.version>14.4.0</vaadin.version>
+        <vaadin.version>14.6.8</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>