we should select a minimal supported version, and run all tests on all bash versions, after the minimal supported one. i think its reasonable to support every bash version after 4.0, as it supports hashmaps which we are likely to implement. it is also since 2009, which means it has broad support on most machines

i suggest we download all bash versions after 4.0 from the bash https server, and then execute tests bwrapped with each version, like this:

run_for_ver() {
    curl https://ftp.gnu.org/gnu/bash/$1.tar.gz
    tar xzf $1.tar.gz
    bwrap --bind $1/bin/bash:/usr/bin/bash cargo r -r
    rm -r $1
}

run_for_ver 4.0
run_for_ver 4.1
...
run_for_ver 5.3