jgvariant-tool: Create multi-release jars.

This is required because Yasson is a multi-release library.  The class
org.eclipse.yasson.internal.ClassMultiReleaseExtension, which adds
record class support for Java 16+, is not available if we do not build
a multi-release jar.

Change-Id: I977a0ad0f7fe0f5a61881ac13f72d5a5280edc95
diff --git a/jgvariant-tool/pom.xml b/jgvariant-tool/pom.xml
index 60db3f3..9b63a06 100644
--- a/jgvariant-tool/pom.xml
+++ b/jgvariant-tool/pom.xml
@@ -102,6 +102,9 @@
             <manifest>
               <mainClass>eu.mulk.jgvariant.tool.Main</mainClass>
             </manifest>
+            <manifestEntries>
+              <Multi-Release>true</Multi-Release>
+            </manifestEntries>
           </archive>
         </configuration>
       </plugin>
@@ -147,6 +150,9 @@
                     <manifest>
                       <mainClass>eu.mulk.jgvariant.tool.Main</mainClass>
                     </manifest>
+                    <manifestEntries>
+                      <Multi-Release>true</Multi-Release>
+                    </manifestEntries>
                   </archive>
                   <descriptorRefs>
                     <descriptorRef>jar-with-dependencies</descriptorRef>