warning BCP037 for missing sku.tier property of type NatGatewaySku

**Bicep version**
Bicep CLI version 0.4.1124 (66c84c8ee5)

**Describe the bug**
warning when setting the sku.tier property of Microsoft.Network/natGateways@2020-11-01

**To Reproduce**
try to create a nat gateway with the tier property set to 'Regional'
```bicep
resource natGateways_nat_outbount_west_name_resource 'Microsoft.Network/natGateways@2020-11-01' = {
name: myregionalnatGw
location: 'westus'
tags: {
environment: 'prod'
role: 'skutest'
}
sku: {
name: 'Standard'
// parser says tier is not allowed
tier: 'Regional'
}
properties: {
idleTimeoutInMinutes: 4
publicIpAddresses: [
{
id: someIp.id
}
]
}
}
```

**Additional context**
whatif tells me the property is being correctly applied.
Warning BCP037: The property "tier" is not allowed on objects of type "NatGatewaySku". No other properties are allowed.

Reproduce the warning with the provided nat gateway Bicep resource and Microsoft.Network/natGateways@2020-11-01 schema. Trace the resource type definition used for NatGatewaySku and verify how the tier property is represented. Done means the Regional tier is accepted without BCP037 while what-if continues to apply it correctly.

Written by the indexing model from the issue text.