build: upgrade vaadin dependency to 21.0.2

Author: javier-godoy

pom.xml

@@ -9,7 +9,7 @@
     <description>Integration of xterm.js for Vaadin Flow</description>
 
     <properties>
-        <vaadin.version>20.0.7</vaadin.version>
+        <vaadin.version>21.0.2</vaadin.version>
 
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>