[Qt] Fix deprecated QButtonGroup::buttonClicked event

Summary: This is replaced by QButtonGroup::idClicked since Qt 5.15.

Test Plan:
With Qt 5.5.1 and 5.15:

ninja all check

Check the warning is gone.

With Qt 5.15, run bitcoin-qt and check the custom fee feature still
works as expected.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6606