Merge #21346: doc: install qt5 when building on macOS

bec7f2c doc: install qt5 when building on macOS (fanquake)

Pull request description:

  Brew has updated such that qt now refers to [Qt 6.0.1](https://github.com/Homebrew/homebrew-core/blob/master/Formula/qt.rb). If builders
  install this, configure will not pick up qt. For now, install
  [qt@5 (5.15.2)](https://github.com/Homebrew/homebrew-core/blob/master/Formula/[email protected]), until required build system and likely source changes
  are made.

ACKs for top commit:
  hebasto:
    ACK bec7f2c, tested on Tested on macOS Big Sur 11.2.2 (20D80).

Tree-SHA512: 86663cfbc68c8c6f5d608d60cd59b37d3faf1e7f33ae17ec2e1a7c076e835eb8200181a17575f121929ea6ecded74b1619096fe5a763106f56de0bdbea9ae4fa

Author: MarcoFalke

doc/build-osx.md

@@ -19,7 +19,7 @@ Then install [Homebrew](https://brew.sh).
 
 ## Dependencies
 ```shell
-brew install automake libtool boost miniupnpc libnatpmp pkg-config python qt libevent qrencode
+brew install automake libtool boost miniupnpc libnatpmp pkg-config python qt@5 libevent qrencode
 ```
 
 If you run into issues, check [Homebrew's troubleshooting page](https://docs.brew.sh/Troubleshooting).