Backup-AzApiManagement: BadRequest

### Description

We have been using `Backup-AzApiManagement` for quite a long time to backup our API Management instance, since this week this script has started failing with this error:

> Operation returned an invalid status code 'BadRequest'

When I add debug logging I find the following information:
``` json
{
"error": {
"code": "InvalidParameters",
"message": "Invalid parameter: Requested value 'GET,HEAD,POST,PUT,DELETE,OPTIONS,PATCH' was not found.\r\nParameter name: backupStorageAccount",
"details": null,
"innerError": null
}
}
```

Nothing on our side has changed as far as I know, what could be causing this issue?

### Issue script & Debug output

```PowerShell
DEBUG: ============================ HTTP REQUEST ============================

HTTP Method:
POST

Absolute Uri:
https://management.azure.com/subscriptions/{subscription}/resourceGroups/{resourcegroup}/providers/Microsoft.ApiManagement/service/{apimname}/backup?api-version=2021-08-01

Headers:
Accept-Language : en-US
x-ms-client-request-id : a050d5b5-6b96-4478-a023-8cb52be674be

Body:
{
"storageAccount": "{storageaccount}",
"containerName": "{containername}",
"backupName": "{backupname}",
"accessType": "AccessKey",
"accessKey": "{token}"
}

DEBUG: ============================ HTTP RESPONSE ============================

Status Code:
BadRequest

Headers:
Cache-Control : no-cache
Pragma : no-cache
Strict-Transport-Security : max-age=31536000; includeSubDomains
Server : Microsoft-HTTPAPI/2.0
x-ms-ratelimit-remaining-subscription-writes: 1199
x-ms-request-id : 9cde0310-e539-455a-b5cd-b8e7bcf85a91
x-ms-correlation-request-id : 9cde0310-e539-455a-b5cd-b8e7bcf85a91
x-ms-routing-request-id : WESTEUROPE:20231114T125043Z:9cde0310-e539-455a-b5cd-b8e7bcf85a91
X-Content-Type-Options : nosniff
Date : Tue, 14 Nov 2023 12:50:42 GMT

Body:
{
"error": {
"code": "InvalidParameters",
"message": "Invalid parameter: Requested value 'GET,HEAD,POST,PUT,DELETE,OPTIONS,PATCH' was not found.\r\nParameter name: backupStorageAccount",
"details": null,
"innerError": null
}
}
```

### Environment data

```PowerShell
Name Value
---- -----
PSVersion 7.3.9
PSEdition Core
GitCommitId 7.3.9
OS Microsoft Windows 10.0.22631
Platform Win32NT
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
WSManStackVersion 3.0
```

### Module versions

```PowerShell
ModuleType Version PreRelease Name ExportedCommands
---------- ------- ---------- ---- ----------------
Script 2.13.2 Az.Accounts {Add-AzEnvironment, Clear-AzConfig, Clear-AzConte…
Script 4.0.2 Az.ApiManagement {Add-AzApiManagementApiToGateway, Add-AzApiManage…
Script 6.0.0 Az.Storage {Add-AzRmStorageContainerLegalHold, Add-AzStorage…
```

### Error output

```PowerShell
HistoryId: 2

Message : Operation returned an invalid status code 'BadRequest'
StackTrace : at Microsoft.Azure.Management.ApiManagement.ApiManagementServiceOperations.BeginBackupWithHttpMessa
gesAsync(String resourceGroupName, String serviceName, ApiManagementServiceBackupRestoreParameters
parameters, Dictionary`2 customHeaders, CancellationToken cancellationToken)
at Microsoft.Azure.Management.ApiManagement.ApiManagementServiceOperations.BackupWithHttpMessagesAs
ync(String resourceGroupName, String serviceName, ApiManagementServiceBackupRestoreParameters
parameters, Dictionary`2 customHeaders, CancellationToken cancellationToken)
at Microsoft.Azure.Management.ApiManagement.ApiManagementServiceOperationsExtensions.BackupAsync(IA
piManagementServiceOperations operations, String resourceGroupName, String serviceName,
ApiManagementServiceBackupRestoreParameters parameters, CancellationToken cancellationToken)
at Microsoft.Azure.Management.ApiManagement.ApiManagementServiceOperationsExtensions.Backup(IApiMan
agementServiceOperations operations, String resourceGroupName, String serviceName,
ApiManagementServiceBackupRestoreParameters parameters)
at Microsoft.Azure.Commands.ApiManagement.ApiManagementClient.BackupApiManagement(String
resourceGroupName, String serviceName, String storageAccountName, String storageAccountKey, String
backupContainer, String backupBlob, String accessType, String identityClientId)
at Microsoft.Azure.Commands.ApiManagement.Commands.BackupAzureApiManagement.ExecuteCmdlet()
at Microsoft.WindowsAzure.Commands.Utilities.Common.CmdletExtensions.<>c__3`1.b__3_0(T c)
at
Microsoft.WindowsAzure.Commands.Utilities.Common.CmdletExtensions.ExecuteSynchronouslyOrAsJob[T](T
cmdlet, Action`1 executor)
at
Microsoft.WindowsAzure.Commands.Utilities.Common.CmdletExtensions.ExecuteSynchronouslyOrAsJob[T](T
cmdlet)
at Microsoft.WindowsAzure.Commands.Utilities.Common.AzurePSCmdlet.ProcessRecord()
Exception : Microsoft.Azure.Management.ApiManagement.Models.ErrorResponseException
InvocationInfo : {Backup-AzApiManagement}
Line : Backup-AzApiManagement -ResourceGroupName "{resourcegroup}" -Name "{apimname}" -StorageContext
$StorageContext -TargetContainerName "{containername}" -TargetBlobName "{backupname}" -verbose -debug
Position : At C:\Users\Pim Simons\Desktop\test.ps1:4 char:1
+ Backup-AzApiManagement -ResourceGroupName "vg-acc-esb-infra" -Name "v …
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
HistoryId : 2
```

Start with the Backup-AzApiManagement entry point and the Microsoft.Azure.Management.ApiManagement backup calls shown in the stack trace. Reproduce the BadRequest using API version 2021-08-01, then trace how backupStorageAccount and accessType are passed to the service. Done means the cause of the parameter rejection and a verified remediation are identified.

Written by the indexing model from the issue text.