azurerm_virtual_network_gateway: increase timeout for create

[wuxu92]

some acctest may failed occasionally with error we below, try to increase the timeout value.

=== CONT  TestAccVirtualNetworkGateway_requiresImport
    testcase.go:113: Step 1/2 error: Error running apply: exit status 1
        Error: waiting for completion of Virtual Network Gateway: (Name "acctestvng-230630050833542667" / Resource Group "acctestRG-230630050833542667"): 
Future#WaitForCompletion: context has been cancelled: StatusCode=200 -- Original Error: context deadline exceeded

=== CONT  TestAccVirtualNetworkGatewayConnection_natRuleIds
    testcase.go:113: Step 1/2 error: Error running apply: exit status 1
        Error: waiting for completion of Virtual Network Gateway: (Name "acctestvnetgw-230814084927775723" / Resource Group "acctestRG-230814084927775723"): Future#WaitForCompletion: context has been cancelled: StatusCode=200 -- Original Error: context deadline exceeded

[stephybun]

Thanks @wuxu92, can you please update the docs as well?

[wuxu92]

@stephybun document updated.

[tombuildsstuff]

LGTM - thanks for this @wuxu92

dismissing since changes have been pushed

[github-actions]

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