do not flatten with OSS mode

the OSS mode despite saying it keeps everything "repositories and
pluginRepositories" does not and it throws away the dependencyManagement

this creates our own descriptor that dies the same as the oss mode
currently but additionally keeps the dependencyManagement section

Whilst we could have used resolveCiFriendliesOnly which is probably a
better solution so that the poms behave better regardless of if they are
deployed from CI or local snapshots this keeps the changes to published
poms minimal

pom.xml

@@ -1313,7 +1313,25 @@
                 </goals>
                 <phase>process-resources</phase>
                 <configuration>
-                  <flattenMode>oss</flattenMode>
+                  <!-- flattenMode:oss does not keep dependecnyMangement or will promote all transitive dependencies to direct ones, changing resolution order -->
+                  <pomElements>
+                    <dependencyManagement>interpolate</dependencyManagement>
+                    <!-- from https://github.com/mojohaus/flatten-maven-plugin/blob/flatten-maven-plugin-1.3.0/src/main/java/org/codehaus/mojo/flatten/FlattenMode.java#L100 -->
+                    <ciManagement>expand</ciManagement>
+                    <contributors>expand</contributors>
+                    <distributionManagement>expand</distributionManagement>
+                    <inceptionYear>expand</inceptionYear>
+                    <issueManagement>expand</issueManagement>
+                    <mailingLists>expand</mailingLists>
+                    <organization>expand</organization>
+                    <prerequisites>expand</prerequisites>
+                    <name>expand</name>
+                    <description>expand</description>
+                    <url>expand</url>
+                    <scm>expand</scm>
+                    <developers>expand</developers>
+                    <repositories>expand</repositories>
+                  </pomElements>
                   <outputDirectory>${project.build.directory}</outputDirectory>
                   <flattenedPomFilename>${project.artifactId}-${project.version}.pom</flattenedPomFilename>
                 </configuration>