Document removal of package-plugin command

[dakshmehta007]

Documents the removal of package-plugin command and provides migration path in UPGRADE7.md

Context

• The package-plugin command was removed in Grails 4.0.4
• Documentation needed updating to reflect this change and provide alternatives
• Issue Seems package-plugin command was not found on version 4.0.4 #11613 reported missing command

Changes

• Added section in UPGRADE7.md about removed command line tools
• Documents the replacement using Gradle standard tasks:
  • ./gradlew assemble for packaging plugins
  • ./gradlew jar for building plugin JARs
• Explains rationale for change (aligning with Gradle standards)

Testing

• No functional testing needed as this is a documentation-only change
• Verified accuracy of Gradle commands for plugin packaging

Additional Notes

• Part of ongoing documentation improvements for Grails 7.0
• Helps users migrate from older Grails versions using legacy commands
• Aligns with modern Gradle-based build practices

Fixes #11613

[jdaugherty]

I love the clean up of this file, but this is our notes file to remind us to update the grails-doc repository (which is what users see). The proper place to update this is https://github.com/apache/grails-doc. Can you submit a PR there?

[jdaugherty]

The specific files to consider updating are:

• https://github.com/apache/grails-doc/blob/7.0.x/src/en/guide/upgrading/upgrading60x.adoc
• https://github.com/apache/grails-doc/blob/7.0.x/src/en/guide/introduction/whatsNew.adoc