Update skymarshal when next version of Dex with Client Credentials flow comes out

Summary

Dex has finally added support for Client Credentials: https://github.com/dexidp/dex/pull/4583

Which is one of THE major reasons for our Dex fork: https://github.com/concourse/dex/tree/pr/client-credentials-grant-sync

The PR isn't in a release of Dex yet. The next release of Dex will have it and we'll likely have to update our fork and code to accommodate this.

We should also see if we can set Public: true on the clients that we register: https://github.com/concourse/concourse/blob/22ccaf2a34385115db24c7e7e0568f1fae85ed0e/skymarshal/dexserver/dexserver.go#L91-L97

Context

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