Update Logstash "rpm/deb upgrade docs" to warn users about reinstalling not officially supported plugins

[edmocosta]

Description:

When Logstash is upgrade via RPM/DEB. It silently removes all non-built-in plugins that have been installed by the user. These plugins need to be reinstalled manually after upgrading.

Updating the Logstash docs to warn users about this behavior, in addition to (#15329), would be helpful to users.

Suggested URLs

• https://www.elastic.co/guide/en/logstash/current/upgrading-logstash.html
• https://www.elastic.co/guide/en/logstash/current/working-with-plugins.html