help-docs: Update Homebrew instructions for the latest release on macOS.
help-docs: Update Homebrew instructions for the latest release on macOS.

The command `brew cask` is no longer a `brew` command as of Homebrew
version 3.5.2.

Updates the instruction to use `brew <command> --cask` instead.

Fixes: #22277.

andersk/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip