This change makes all the pre-commit CI scripts runnable under Bash 3.2,
by replacing “mapfile” invocations in them code that first explicitly
creates an array, and then uses a while loop to populate the array.
Otherwise, without this change, the scripts all fail to run under Bash
3.2 — due to lack of support for “mapfile”.
Fixes https://github.com/LadybirdBrowser/ladybird/issues/283
This also drops bash from the list of homebrew dependencies in the build
instructions — because with this change, homebrew bash (v4) is no longer
needed; things will now work with the Apple-provided bash (v3.2)
