build: move release-only plugins to release profile

[marevol]

Summary

Standardize the release configuration following the maven-release-plugin 2→3 migration guide (https://maven.apache.org/maven-release/maven-release-plugin/migrate.html): release-only plugins now run via the release profile, which maven-release-plugin activates automatically during release:perform.

Changes Made

• Added maven-release-plugin 3.1.1 with <releaseProfiles>release</releaseProfiles> to the parent build
• Moved the maven-source-plugin / maven-javadoc-plugin references from the core and toolkit module builds into the parent release profile (versions and executions are still managed in pluginManagement), so sources/javadoc jars are generated only during releases, alongside GPG signing

Testing

• mvn package -DskipTests — no sources/javadoc jars produced in any module, no GPG required for local builds
• mvn package -P release -DskipTests -Dgpg.skip=true — core and toolkit both produce sources and javadoc jars

Breaking Changes

• None (release artifacts are unchanged)

Additional Notes

• Plain mvn install no longer attaches sources/javadoc jars locally; they are generated only during releases.
• Same pattern as codelibs/fesen-httpclient; applied across CodeLibs repositories for consistency.