doc: install qt5 when building on macOS
Summary:
Brew has updated such that qt now refers to Qt 6.0.1. If builders install this, configure will not work pick up qt. For now, install qt@5 (5.15.2), until required build system and likely source changes are made.
Backport of core#21346.
Test Plan:
On OSX:
brew update brew info qt # returns a qt 6.x.x version brew info qt@5 # returns a qt 5.x.x version
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D9317