Update qt/bitcoinamountfield.cpp to use Qt5 connect syntax

Summary:
Updates to use Qt5 connect syntax over Qt4.
See https://wiki.qt.io/New_Signal_Slot_Syntax for details.

Partial backport of Core PR13529
https://github.com/bitcoin/bitcoin/pull/13529/commits/f78558f1e39198779bdb17e2b0e256fb99ad4b2

Test Plan:

make check
./bitcoin-qt
Send

Use the increment and decrement buttons to increase and decrease the value in the amount field. Then change the number directly.

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4310