elaborate the migration startup message

[aryairani]

Currently:

📋 I backed up your codebase to /Users/arya/.unison/v2/unison.sqlite3.v15.1715812947
⚠️  Please close all other ucm processes and wait for the migration to complete before interacting with your codebase.
Press <enter> to start the migration once all other ucm processes are shutdown...

Draft proposal:

Your codebase format is version 15; I will upgrade it to version 16.

📋 I backed up your codebase to /Users/arya/.unison/v2/unison.sqlite3.v15.1715812947
⚠️  Please close all other ucm processes and wait for the migration to complete before interacting with your codebase.
Press <enter> to start the migration once all other ucm processes are shutdown...

cc @hojberg @rlmark for emoji suggestions :)

[aryairani]

Alternatively if we come up with some other way to do a clean reboot of the UCMs without manual intervention, that'd be even better.

[hojberg]

I'd maybe suggest a ✅ for the backup line, but I don't have any strong feelings. What you have seems good also (I'm surprised there's not a safe or a vault emoji).