Update `codegen-library.sh` to set exec flag on new k8s.io/code-generator scripts

As of `v0.28.0` of k8s.io/code-generator the `generate-groups.sh` script calls `generate-internal-groups.sh`. We should update our script to additionall add the exec flag for the `generate-internal-groups.sh` file.

https://github.com/kubernetes/code-generator/blob/791c213a776b4b92f9af0805c1cf57c118ba951c/generate-groups.sh#L66

Start in codegen-library.sh and compare its handling of generate-groups.sh with the linked Kubernetes code-generator change. Confirm how generate-groups.sh invokes generate-internal-groups.sh, update the script so the new file receives the executable flag, and verify the generation flow completes with the resulting script executable.

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