chore: Save the date when the country is changed

[g123k]

Hi everyone!

As we wait for 4.15.0, I'll add a small change I will use later.
It saves the date when the country is changed.

This will allow us to display a message when the user is abroad.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 7.18%. Comparing base (4d9c7fc) to head (8cafb7d).
Report is 229 commits behind head on develop.

Files	Patch %	Lines
.../lib/data_models/preferences/user_preferences.dart	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #5443      +/-   ##
==========================================
- Coverage     9.54%   7.18%   -2.37%     
==========================================
  Files          325     391      +66     
  Lines        16411   20382    +3971     
==========================================
- Hits          1567    1464     -103     
- Misses       14844   18918    +4074     

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[teolemon]

thanks @g123k