Mysql Flexible server admin user does not support hyphen character

After [in-place auto-migration ](https://learn.microsoft.com/en-us/azure/mysql/migrate/migrate-single-flexible-in-place-auto-migration) from single to Flexible server, the `Server admin login name` moved to the Flexible one containing hyphen (-).

Now, we need to import the migrated Flexible server to the Terraform.

The validation is happening on the API side because I am not able to create the sample server with the hyphen:

`az mysql flexible-server create --location westeurope --resource-group test-rg --name example1232453534653 --admin-user "with-hyphen"`
```
(InvalidParameterValue) Invalid value given for parameter administratorLogin. Specify a valid parameter value.
Code: InvalidParameterValue
Message: Invalid value given for parameter administratorLogin. Specify a valid parameter value.
```

This issue is blocking:
https://github.com/hashicorp/terraform-provider-azurerm/issues/26658

Start by reproducing the reported `az mysql flexible-server create` command with an administrator login containing a hyphen and compare the API response with the migrated Flexible server behavior described in the issue. Trace the administratorLogin validation entry point if this repository exposes it; done means establishing whether valid migrated names are accepted or documenting the required service-side change, with the linked Terraform issue no longer blocked.

Written by the indexing model from the issue text.