stdenv: bump required Bash version from 4 to 5

Currently stdenv requires Bash 4.x that was released in 2009. This
change bumps the required version to Bash 5.x (2019, 5 years ago).

See https://mywiki.wooledge.org/BashFAQ/061 for more details.

Using a relatively modern Bash version allows us to rely on newer
features (e.g. ${var@a}) and remove workarounds for older quirks (e.g.
https://stackoverflow.com/a/7577209, “old bash empty array problem”).
Note that many setup hooks are using features added after 4.0 version,
e.g. makeWrapper uses ${var@Q} from 4.4, but some even require >5.0,
e.g. cargoBuildHook uses ${var@U} from 5.1.

pkgs/stdenv/generic/setup.sh

@@ -4,9 +4,9 @@ __nixpkgs_setup_set_original=$-
 set -eu
 set -o pipefail
 
-if [[ -n "${BASH_VERSINFO-}" && "${BASH_VERSINFO-}" -lt 4 ]]; then
+if [[ -n "${BASH_VERSINFO-}" && "${BASH_VERSINFO-}" -lt 5 ]]; then
     echo "Detected Bash version that isn't supported by Nixpkgs (${BASH_VERSION})"
-    echo "Please install Bash 4 or greater to continue."
+    echo "Please install Bash 5 or greater to continue."
     exit 1
 fi