Add flatten-maven-plugin for Sonatype Central Portal (#284)

Sonatype Central Portal validates deployed POMs as standalone files,
unlike the old OSSRH/Nexus which resolved Maven properties during
validation. This caused the deploy to fail because child module POMs
contain unresolved
`${project.groupId}`/`${project.version}` and lack inherited metadata
(url, licenses, scm, developers).

This adds `flatten-maven-plugin` with `flattenMode=ossrh` to the release
profile. This produces self-contained POMs with resolved coordinates,
versions, and inlined parent metadata.

Fixes
https://github.com/openzipkin/zipkin-reporter-java/actions/runs/23481323296/job/68325597619

Signed-off-by: Adrian Cole <adrian@tetrate.io>

Author: codefromthecrypt

.gitignore

@@ -12,6 +12,7 @@ hs_err_pid*
 
 # Maven
 target/
+.flattened-pom.xml
 
 # IntelliJ
 .idea/

pom.xml

@@ -69,6 +69,7 @@
 
     <license.skip>${skipTests}</license.skip>
 
+    <flatten-maven-plugin.version>1.7.3</flatten-maven-plugin.version>
     <go-offline-maven-plugin.version>1.2.8</go-offline-maven-plugin.version>
     <!-- TODO: cleanup any redundant ignores now also in the 4.0 release (once final) -->
     <license-maven-plugin.version>5.0.0</license-maven-plugin.version>
@@ -292,6 +293,12 @@
           <version>${central-publishing-maven-plugin.version}</version>
         </plugin>
 
+        <plugin>
+          <groupId>org.codehaus.mojo</groupId>
+          <artifactId>flatten-maven-plugin</artifactId>
+          <version>${flatten-maven-plugin.version}</version>
+        </plugin>
+
         <plugin>
           <groupId>org.eclipse.m2e</groupId>
           <artifactId>lifecycle-mapping</artifactId>
@@ -578,6 +585,30 @@
       <id>release</id>
       <build>
         <plugins>
+          <plugin>
+            <groupId>org.codehaus.mojo</groupId>
+            <artifactId>flatten-maven-plugin</artifactId>
+            <configuration>
+              <flattenMode>ossrh</flattenMode>
+            </configuration>
+            <executions>
+              <execution>
+                <id>flatten</id>
+                <phase>process-resources</phase>
+                <goals>
+                  <goal>flatten</goal>
+                </goals>
+              </execution>
+              <execution>
+                <id>flatten.clean</id>
+                <phase>clean</phase>
+                <goals>
+                  <goal>clean</goal>
+                </goals>
+              </execution>
+            </executions>
+          </plugin>
+
           <plugin>
             <groupId>org.sonatype.central</groupId>
             <artifactId>central-publishing-maven-plugin</artifactId>