[TECH DEBT] Remove DefaultAzureCredential Fallback Option

[shirasassoon]

This will be a Breaking Change in fabric-cicd. Due to security best practices, the library will no longer support running a deployment without providing an explicit token_credential parameter.

Phase 1: Updated documentation with deprecation notices and authentication guidance has already been published (done)
Phase 2: Remove DefaultAzureCredential Fallback logic from library code (to do - breaking)

[microsoft-github-policy-service]

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