docs: recommend upgrading macOS

Recommend that macOS users upgrade their bash.

docs/developer-setup.md

@@ -38,9 +38,14 @@ Linux users can follow the instructions for their distro on that site. For Macos
 # Ensure that xcode build tools are installed
 xcode-select --install
 # Ensure required libraries for python
-brew install openssl readline sqlite3 xz zlib
+brew install openssl readline sqlite3 xz zlib bash
 ```
+**NOTE for macOS Users**: in the previous command we also installed a modern version of `bash`.
+This is because macOS ships with an very outdated version (circa 2007) of `bash`, due to licensing issues.
+Homebrew should install a modern version (at least v5).
+If scripts are failing on macOS, check that the Homebrew copy of bash is on the path, eg: `/opt/homebrew/bin/bash`.
+
 ### Step 2.5: Apple Silicon users only: install Rosetta
 > To determine if you are using an Apple with Silicon or an Intel-based processor, select the **Apple** menu icon in the top-left of the desktop and click **About This Mac**. On Mac computers with Apple silicon, About This Mac shows an item labeled **Chip**, followed by the name of the chip (ex. **Chip** Apple M1 Pro). On Mac computers with an Intel processor, About This Mac shows an item labeled **Processor**, followed by the name of an Intel processor.