azurerm_mssql_server : fix azurerm_mssql_server automatically sets minimum_tls_version even if not provided issue

[sinbai]

In PR #14229, the minimum_tls_version property of resource azurerm_mssql_server was added with a default value. This means using terraform to create resource azurerm_mssql_server the TLS version is enforced by default when the user does not specify a value for minimum_tls_version , and it could no longer be reverted. After verifying the behavior of the creating Sql Server API, if minimum_tls_version is not specified , all TSL versions are allowed by default. Also, when creating a sql server through Azure portal, it behaviors the same as the API. So, adding Disabled value make user explicitly opt TLS version to fix issue #16565.