upgraded Qt to 6 for nano_wallet

[jamescoxon]

(originally released in 2020), while the last version of Qt5 will have support for 3 years for commercial licence users increasingly there will be a move to Qt6 - for example for M1 silicon only Qt6 is available (for now https://www.qt.io/blog/qt-on-apple-silicon)

also important to recognise that nano_wallet is rarely used these days due to better community gui wallets.

Main change is making Qt references in CMakeList.txt versionless

[dsiganos]

Abandoning this Pr because we do not need QT6. We were upgrading to Qt6 because we thought Qt6 was necessary to work on Apple M1.

However, the following appears to work on Apple M1.

brew install qt@5
brew link qt@5

Then adjust PATH in .zrcsh as suggested by brew link command:

echo 'export PATH="/opt/homebrew/opt/qt@5/bin:$PATH"' >> ~/.zshrc