Storage Account: mark location with x-ms-mutability

Storage Account's [`location`](https://github.com/Azure/azure-rest-api-specs/blob/27cc07ddd294d98e05cb301e07a72378df9f87e8/specification/storage/resource-manager/Microsoft.Storage/stable/2019-06-01/storage.json#L2206) property should be marked with `x-ms-mutability` attribute to annotate that it's non-updatable:

```
"location": {
"type": "string",
"description": "Required. Gets or sets the location of the resource. This will be one of the supported and registered Azure Geo Regions (e.g. West US, East US, Southeast Asia, etc.). The geo region of a resource cannot be changed once it is created, but if an identical geo region is specified on update, the request will succeed.",
"x-ms-mutability": [
"create",
"read"
]
},
```

Open the linked storage.json specification at the Storage Account location property around line 2206. Compare its metadata with the requested x-ms-mutability create/read annotation, then verify that the property is marked non-updatable as described in the issue.

Written by the indexing model from the issue text.