fix: pom.xml to reduce vulnerabilities (#113)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-COMVAADIN-1085644

Co-authored-by: snyk-bot <snyk-bot@snyk.io>

pom.xml

@@ -59,7 +59,7 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <vm.version>${project.version}</vm.version>
-        <vaadin.version>7.7.13</vaadin.version>
+        <vaadin.version>7.7.23</vaadin.version>
         <netbeans.version>RELEASE82</netbeans.version>
         <junit.version>4.12</junit.version>
         <mysql.version>8.0.16</mysql.version>