qt6: Handle deprecated QLocale::nativeCountryName

Summary:
This change ensures compatibility across all supported Qt versions.

This is a backport of core-gui#838
Depends on D18198

Test Plan: compiled with qt 5 and qt 6.4.2

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18199