Azure SQL Server: External AAD Administrator has incorrect mutability

The [spec for Microsoft.Sql/servers](https://github.com/Azure/azure-rest-api-specs/blob/main/specification/sql/resource-manager/Microsoft.Sql/preview/2022-02-01-preview/Servers.json#L971-L978) lists read and create under x-ms-mutability.

API does support updates through this endpoint, since assigning an External Active Directory user is possible through [Azure PowerShell](https://docs.microsoft.com/en-us/powershell/module/az.sql/set-azsqlinstanceactivedirectoryadministrator?view=azps-8.2.0) and the Azure Portal without re-creating the resource.

Related [Issue](https://github.com/Azure/azure-rest-api-specs/issues/16803)

Pulumi [issue](https://github.com/pulumi/pulumi-azure-native/issues/1738) also related.

This issue is also probably the cause of this [issue](https://github.com/Azure/azure-rest-api-specs/issues/16423)

Open specification/sql/resource-manager/Microsoft.Sql/preview/2022-02-01-preview/Servers.json at lines 971-978 and inspect the External AAD Administrator property's x-ms-mutability against the update behavior described in the issue. Done means the specification permits the supported update operation and passes the repository's applicable specification validation.

Written by the indexing model from the issue text.