[FLINK-12598][build] Use shade-plugin 3.1.1 for shading sources

This fixes a potential NPE (seems to appear only with some maven versions) when
trying to shade sources with a dependency being unavailable.

pom.xml

@@ -198,6 +198,7 @@ under the License.
                         <plugin>
                             <groupId>org.apache.maven.plugins</groupId>
                             <artifactId>maven-shade-plugin</artifactId>
+                            <version>3.1.1</version>
                             <configuration combine.children="append">
                                 <createSourcesJar>true</createSourcesJar>
                                 <shadeSourcesContent>true</shadeSourcesContent>