CAY-2996 Move source and javadoc jar generation to a release profile - #651

What does this PR do?

Moves source and Javadoc JAR generation into a dedicated release Maven profile.

Normal Maven builds now produce only the main and test artifacts. When the release profile is enabled, Maven also attaches -sources.jar and -javadoc.jar for publication.

Previously, source JAR generation was enabled for every Maven build, while Javadoc JAR generation was not configured as part of the Maven release artifact lifecycle.

After the change, the release guide at https://cayenne.apache.org/dev/release-guide.html should use the release profile:

 cd cayenne
 mvn release:clean
 mvn release:prepare -DpreparationGoals="clean install" -DautoVersionSubmodules=true
-mvn release:perform -P gpg [-Dgpg.keyname=B8AF90BF]
+mvn release:perform -P release [-Dgpg.keyname=B8AF90BF]

Testing

Verified that:

mvn clean install -DskipTests

does not generate or install source or Javadoc JARs.

Verified that:

mvn clean install -Prelease -DskipTests

generates and installs:

*-sources.jar
*-javadoc.jar