ci: configure Javadoc failOnWarnings

Close #85

Author: javier-godoy

pom.xml

@@ -307,7 +307,7 @@
  <configuration>
  <quiet>true</quiet>
  <doclint>none</doclint>
- <failOnWarnings>false</failOnWarnings>
+ <failOnWarnings>true</failOnWarnings>
  <links>
  <link>https://javadoc.io/doc/com.vaadin/vaadin-platform-javadoc/${vaadin.version}</link>
  </links>