Remove reference to AZURE_KEYVAULT_AUTH_VIA_CLI (#12514)

The environment variable AZURE_KEYVAULT_AUTH_VIA_CLI was removed in google/go-cloud#3202.

Co-authored-by: Troy Howard <[email protected]>

content/docs/iac/concepts/secrets.md

@@ -561,7 +561,7 @@ $ pulumi stack init my-stack \
     --secrets-provider="azurekeyvault://acmecorpsec.vault.azure.net/keys/payroll"
 ```
 
-By default, this provider will use [Azure Environment Authentication](https://docs.microsoft.com/en-us/azure/go/azure-sdk-go-authorization#use-environment-based-authentication). If you wish to login using the `az` command for authentication instead, set `AZURE_KEYVAULT_AUTH_VIA_CLI` to `"true"` (using double quotes).
+This provider will attempt to use [a series of authentication methods](https://pkg.go.dev/github.com/Azure/azure-sdk-for-go/sdk/azidentity#readme-defaultazurecredential).
 
 #### Google Cloud Key Management Service (KMS)