Support older bash versions

[tinvaan]

Looks like the ^^ parameter expansion is not supported in earlier versions of bash, resulting in a failure when running the generate-client command.

harish at busybox in ~/Workspaces/oss/pulp/oci_env on setup/osx
(pulpcore) ± bash --version
GNU bash, version 3.2.57(1)-release (arm64-apple-darwin24)
Copyright (C) 2007 Free Software Foundation, Inc.

harish at busybox in ~/Workspaces/oss/pulp/oci_env on setup/osx
(pulpcore) ± oci-env generate-client -i
2025-01-19 22:39:17,260 - INFO - utils.py:17 - USING OCI_ENV_PATH FROM ENV: /Users/harish/Workspaces/oss/pulp/oci_env
2025-01-19 22:39:17,260 - INFO - utils.py:17 - USING OCI_ENV_PATH FROM ENV: /Users/harish/Workspaces/oss/pulp/oci_env
2025-01-19 22:39:17,260 - INFO - utils.py:17 - USING OCI_ENV_PATH FROM ENV: /Users/harish/Workspaces/oss/pulp/oci_env
Generating python-client for pulpcore.
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 1017k  100 1017k    0     0  2007k      0 --:--:-- --:--:-- --:--:-- 2006k
::group::BINDINGS
./gen-client.sh: line 57: OPENAPI_${LANGUAGE^^}_IMAGE: bad substitution