chore(deps): upgrade maven version to fix integ test (#211)

Author: janell-chen

functions-framework-api/pom.xml

@@ -151,7 +151,7 @@
           <plugin>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-source-plugin</artifactId>
-            <version>3.3.0</version>
+            <version>3.6.0</version>
             <executions>
               <execution>
                 <id>attach-sources</id>
@@ -203,4 +203,4 @@
       </build>
     </profile>
   </profiles>
-</project>
+</project>