Don't require CM "mediation" to be specified explicitly (always 'required' for DC).

[samuelgoto]

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Given #175, we are going to need to update every example and deployment to use mediation="required".

A few examples are, since this is backwards incompatible change:

1. https://openid.github.io/OpenID4VP/openid-4-verifiable-presentations-wg-draft.html#name-request
2. https://digitalcredentials.dev/
3. https://developer.chrome.com/blog/digital-credentials-api-origin-trial

And every demo app that was built against our API.