feat: Remove refresh_token grant type from automated refresh

[JorTurFer]

SA key flow is designed for machine 2 machine communication, so using refresh_token doesn't make sense from operational pov as the sessions doesn't need to be extended (like it happens for users that you want to silent refresh).

Using refresh_token grant type it's not needed because for a workload, using the jwt-bearer grant type or refresh token grant type is exactly the same, but SA's refresh_token grant type isn't OAuth compliant as it doesn't require client_id (which is mandatory).

I've tested that everything works running the backgroundrefresh example

Checklist

• No generated code was adjusted manually (check comments in file header)
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• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
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• Unit tests are passing: make test (will be checked by CI)
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