fix: Update maven-exec-plugin to 1.6.0

pom.xml

@@ -20,13 +20,12 @@
   </prerequisites>
 
   <properties>
+    <!-- source file encoding -->
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <file.encoding>UTF-8</file.encoding>
 
     <!-- Project properties -->
     <javaVersion>11</javaVersion>
-    <signatureArtifactId>java18</signatureArtifactId>
-    <signatureVersion>1.0</signatureVersion>
 
     <!-- Commons versions -->
     <duniter4j.version>1.5.10</duniter4j.version>
@@ -109,7 +108,7 @@
     <!-- I18n configuration -->
     <i18n.bundles>fr_FR,en_GB</i18n.bundles>
     <i18n.silent>true</i18n.silent>
-    
+
     <!-- by default, use maven 2 source base dir -->
     <maven.src.dir>${basedir}/src</maven.src.dir>
 
@@ -475,8 +474,7 @@
         <plugin>
           <groupId>org.codehaus.mojo</groupId>
           <artifactId>exec-maven-plugin</artifactId>
-          <version>1.5.0</version>
-
+          <version>${execPluginVersion}</version>
         </plugin>
 
         <plugin>