Update Debian version to 12 in systemvm welcome message

[weizhouapache]

Description

as a followup of #8497

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)
• build/CI
• test (unit or integration test code)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• Major
• Minor

Bug Severity

• BLOCKER
• Critical
• Major
• Minor
• Trivial

Screenshots (if appropriate):

How Has This Been Tested?

How did you try to break this feature and the system with this change?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 15.57%. Comparing base (af607b9) to head (2fd137d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #9627 +/- ##
===========================================
Coverage 15.57% 15.57% - Complexity 12048 12049 +1 
===========================================
Files 5505 5505 Lines 482631 482631 Branches 58686 62445 +3759 ===========================================
+ Hits 75185 75186 +1 + Misses 399141 399140 -1 
Partials 8305 8305 

Flag	Coverage Δ
uitests	4.16% <ø> (ø)
unittests	16.35% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sureshanaparti]

LGTM