[QT] Fix QByteArray.append(const QString) deprecation warning in 5.15.1
[QT] Fix QByteArray.append(const QString) deprecation warning in 5.15.1

Summary:
https://doc.qt.io/qt-5/qbytearray-obsolete.html#append-5
This function was implicitely calling `toUtf8()` to convert the string
to a byte array, so this diff does not change the behavior.

Test Plan:
  ninja all check-all
Check the deprecation warning is gone at build time.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7435

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