WEBSITE_CONTENTAZUREFILECONNECTIONSTRING as keyvault reference makes deployment fail

```
Deployment failed. Correlation ID: f171c78c-ad28-4426-aea4-cd9ae0bf57e9. {
"Code": "BadRequest",
"Message": "The parameter 'WEBSITE_CONTENTAZUREFILECONNECTIONSTRING' has an invalid value. Details: Cannot specify key vault references not referencing User Assigned Identity on Create Site.",
"Target": null,
"Details": [
{
"Message": "The parameter 'WEBSITE_CONTENTAZUREFILECONNECTIONSTRING' has an invalid value. Details: Cannot specify key vault references not referencing User Assigned Identity on Create Site."
},
{
"Code": "BadRequest"
},
{
"ErrorEntity": {
"ExtendedCode": "01033",
"MessageTemplate": "The parameter '{0}' has an invalid value. Details: {1}.",
"Parameters": [
"WEBSITE_CONTENTAZUREFILECONNECTIONSTRING",
"Cannot specify key vault references not referencing User Assigned Identity on Create Site"
],
"Code": "BadRequest",
"Message": "The parameter 'WEBSITE_CONTENTAZUREFILECONNECTIONSTRING' has an invalid value. Details: Cannot specify key vault references not referencing User Assigned Identity on Create Site."
}
}
],
"Innererror": null
}
```

Per comment here:
https://github.com/Azure/azure-functions-host/issues/5306#issuecomment-696561132

The payload names no files, tests, or entry points. Start by reading the referenced comment in azure-functions-host issue #5306 and compare its context with the reported deployment error; the issue does not specify the expected behavior or a concrete completion condition.

Written by the indexing model from the issue text.