Context from https://github.com/bitcoin-core/gui/issues/578:
> The most valued benefit will come when migrating to Qt 6, where QRegExp requires usage of an additional Core5Compat module.
> Instead, all use cases of QRegExp could be ported to QRegularExpression.
> qt , test: Add tests for searching in `AddressBookPage` dialog
> qt: Use `QRegularExpression` in `AddressBookSortFilterProxyModel` class
>
> Co-authored-by: João Barbosa <joao.paulo.barbosa@gmail.com>
This is a backport of core#593
Depends on D18176