fix: use a version of maven-source-plugin that's available in mavencentral (#288)

Author: jrmfg

functions-framework-api/pom.xml

@@ -151,7 +151,7 @@
           <plugin>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-source-plugin</artifactId>
-            <version>3.6.0</version>
+            <version>3.3.1</version>
             <executions>
               <execution>
                 <id>attach-sources</id>