Combine minimal-versions scripts for CI

We have a couple of minimal versions scripts that could be combined.

- minimal-versions.sh
- minimal-versions-direct.sh
- workspace-minimal-versions.sh

See comments also https://github.com/googleapis/google-cloud-rust/pull/4414#discussion_r2733842046 https://github.com/googleapis/google-cloud-rust/pull/4410#discussion_r2734218443