Bump maven-source-plugin to 3.4.0

https://github.com/apache/maven-source-plugin/releases/tag/maven-source-plugin-3.4.0

JIRA: LIGHTY-415
Signed-off-by: tobias.pobocik <tobias.pobocik@pantheon.tech>

Author: Tobianas

lighty-core/lighty-parent/pom.xml

@@ -152,7 +152,7 @@
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-source-plugin</artifactId>
-                    <version>3.3.1</version>
+                    <version>3.4.0</version>
                     <executions>
                         <execution>
                             <id>attach-sources</id>