[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