build: upgrade Vaadin version to 14.8.20

Author: javier-godoy

pom.xml

@@ -9,7 +9,7 @@
     <description>Integration of google-map for Vaadin platform</description>
 
     <properties>
-        <vaadin.version>14.7.5</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>