`azure.mgmt.resource.subscriptions` lacks `api_version` 2020-01-01

- **Package Name**: `azure.mgmt.resource.subscriptions`
- **Package Version**: latest

**Describe the bug**
The REST API document for Tenants - List uses `api-version=2020-01-01`:

https://docs.microsoft.com/en-us/rest/api/resources/tenants/list?tabs=HTTP


However, this API version is missing from Python SDK:

https://github.com/Azure/azure-sdk-for-python/blob/0492dc711c69cacc0ec8aea69e7f2d33d0e66c0e/sdk/resources/azure-mgmt-resource/azure/mgmt/resource/subscriptions/_subscription_client.py#L170-L182

The swagger for `2020-01-01` does have `Tenants_List`:

https://github.com/Azure/azure-rest-api-specs/blob/c03c258c7a01a7d57b3110cc20e2e76752b6f2d6/specification/resources/resource-manager/Microsoft.Resources/stable/2020-01-01/subscriptions.json#L156-L162

```
"/tenants": {
"get": {
"tags": [
"Tenants"
],
"operationId": "Tenants_List",
"description": "Gets the tenants for your account.",
```

Another question not belonging to Python SDK: Why isn't the REST API doc updated to use `api-version=2021-01-01` which also has `tenants` API?

https://github.com/Azure/azure-rest-api-specs/blob/c03c258c7a01a7d57b3110cc20e2e76752b6f2d6/specification/resources/resource-manager/Microsoft.Resources/stable/2021-01-01/subscriptions.json#L160-L166

The same happens to `subscriptions`.

Start with the linked azure/mgmt/resource/subscriptions/_subscription_client.py section and the 2020-01-01 and 2021-01-01 subscriptions.json files, comparing their Tenants_List and subscriptions operations. Verify how the REST specification maps to the Python SDK's supported API versions; done means the requested version is consistently represented or the version discrepancy is resolved.

Written by the indexing model from the issue text.