ci: downgrade vaadin version to LTS

Author: mlopezFC

pom.xml

@@ -9,7 +9,7 @@
     <description>Integration of app-layout for Vaadin Flow</description>
 
     <properties>
-        <vaadin.version>18.0.3</vaadin.version>
+        <vaadin.version>14.6.5</vaadin.version>
 
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>