Strengthen auth linter to require managed identity by default

### Clear and concise description of the problem

We should strengthen https://github.com/Azure/typespec-azure/blob/main/packages/typespec-azure-core/src/rules/auth-required.ts to require managed identity for all services and suppress the warning for anything brownfield. We need to make a corresponding change to Azure's guidelines tracked at https://github.com/microsoft/api-guidelines/issues/544.

### Checklist

- [X] Follow our [Code of Conduct](https://github.com/azure/typespec-azure/blob/main/CODE_OF_CONDUCT.md)
- [X] Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the [TypeSpec repo](https://github.com/Microsoft/TypeSpec/issues/new/choose)
- [X] Read the [docs](https://azure.github.io/typespec-azure/).
- [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Start with packages/typespec-azure-core/src/rules/auth-required.ts and inspect how the auth linter currently handles managed identity and brownfield services. Review the corresponding Azure guidelines discussion in microsoft/api-guidelines issue 544; done means the linter behavior and guideline change consistently require managed identity for new services while suppressing the warning for brownfield work.

Written by the indexing model from the issue text.