UX issue: "EOL Rebase" should keep the old .desktop entry to notify users to update their taskbar/desktop/etc items

[rany2]

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Suggestion

Continuation of #5782.

Users have emailed me about accepting the "EOL Rebase" prompt but not being able to access Ungoogled Chromium anymore. The reason is that they are trying to access it from the taskbar/desktop which uses the out-of-date com.github.Eloston.UngoogledChromium app ID.

I propose that the old .desktop item is kept and that flatpak run com.github.Eloston.UngoogledChromium (i.e., the old app ID) notifies the user to update any entries that might use the old app ID. An option will provided so that flatpak would delete the old entries once the user confirms that they've migrated everything.