`Microsoft.DataProtection/BackupVaults/backupPolicies` is not idempotent

**Bicep version**
This is an ARM bug which I don't know where to report properly, but needs action.

**Describe the bug**
`Microsoft.DataProtection/BackupVaults/backupPolicies@2020-01-01` is not idempotent. It errors out with `Status Message: Update of existing policy is not supported. (Code:UserErrorBMSUpdatePolicyNotSupported)`

**To Reproduce**
Steps to reproduce the behaviour would be something similar like this (ARM):
```arm
"type": "Microsoft.DataProtection/BackupVaults/backupPolicies",
"apiVersion": "2020-01-01-alpha",
"name": "[concat(variables('vaultNameRegion'),'/',parameters('policyName'))]",
"dependsOn": [
"[resourceId('Microsoft.DataProtection/BackupVaults', variables('vaultNameRegion'))]"
],
"properties":
{
"policyRules": [
{
"name": "Default",
"objectType": "AzureRetentionRule",
"isDefault": true,
"lifecycles": [
{
"deleteAfter": {
"duration": "P30D",
"objectType": "AbsoluteDeleteOption"
},
"sourceDataStore": {
"dataStoreType": "OperationalStore",
"objectType": "DataStoreInfoBase"
},
"targetDataStoreCopySettings": []
}
]
}
],
"datasourceTypes": [
"Microsoft.Storage/storageAccounts/blobServices"
],
"objectType": "BackupPolicy",
"name": "[parameters('policyName')]"
}
}
```

**Additional context**
Reported [here](https://docs.microsoft.com/en-us/answers/questions/468053/azure-backup-vault-azure-blob-policy.html) as well.

Fixing it with a feature flag or PowerShell is not a solution imho, providing Bicep with an option to check if this resource exists and conditionally create it is.

Start by deploying the supplied ARM template for Microsoft.DataProtection/BackupVaults/backupPolicies@2020-01-01-alpha twice and compare the failure with the Azure Answers report. Determine whether the behavior is actionable in Bicep or belongs to Azure Resource Manager; done should be a documented, supported path for repeated deployment without the update-policy error.

Written by the indexing model from the issue text.