[Task] Include DISCLAIMER in JAR META-INF for incubating artifacts

These four issues capture the non-blocking follow-up items raised by Justin Mclean (IPMC) on the 2.1.0-incubating IPMC vote thread (https://lists.apache.org/thread/6m0vhnc3zknb58t6rkg3wlpt2bp6rg5r).

**Background**

ASF incubating project conventions expect the DISCLAIMER to ship inside the release binaries, not only at the source-tree root. In the currently-voting 2.1.0-incubating staging repository, the published JARs' META-INF/ does not include the DISCLAIMER.

**Task**

Configure the build so the root DISCLAIMER is packaged into each module JAR's META-INF/. Recommended approach:
```
<properties>
  <maven.jar.plugin.version>3.x.x</maven.jar.plugin.version>
</properties>
...
<build>
  <plugins>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-jar-plugin</artifactId>
      <configuration>
        <archive>
          <manifestFiles>
            <manifestFile>${project.basedir}/../DISCLAIMER</manifestFile>
          </manifestFiles>
        </archive>
      </configuration>
    </plugin>
  </plugins>
</build>
```
(Verify path resolution per module; alternatively copy the root DISCLAIMER into each module's src/main/resources/META-INF/.) Also verify the fesod-sources / shaded JARs as appropriate.

Validate on the next RC: confirm META-INF/DISCLAIMER is present in every published JAR.

[]