Bugfix: honor upstream default for enableDynamicPortAllocation

Having a hardcoded default_value of `false` for the
`enableDynamicPortAllocation` property is troublesome, and the
upstream default should be used. This was added as part of GoogleCloudPlatform#6022.

By hardcoding the default here, any client will explicitly deprogram
the dynamic port allocation setting if it is not part of the client
configuration. This is causing problems for those clients that have
enabled the dynamic port allocation out of band, since the current
support is incomplete (not all parameters can be set via Terraform:
terraform-google-modules/terraform-google-cloud-nat#64 (comment)). While
full support is being added, removing hardcoded defaults allow
out-of-band configurations to continue working.

mmv1/products/compute/api.yaml

@@ -13479,7 +13479,6 @@ objects:
           If minPortsPerVm is not set, a minimum of 32 ports will be allocated to a VM from this NAT config.
 
           Mutually exclusive with enableEndpointIndependentMapping.
-        default_value: false
       - !ruby/object:Api::Type::Integer
         name: udpIdleTimeoutSec
         description: |

mmv1/products/compute/terraform.yaml

@@ -2510,6 +2510,8 @@ overrides: !ruby/object:Overrides::ResourceOverrides
         custom_flatten: 'templates/terraform/custom_flatten/default_if_empty.erb'
       tcpTransitoryIdleTimeoutSec: !ruby/object:Overrides::Terraform::PropertyOverride
         custom_flatten: 'templates/terraform/custom_flatten/default_if_empty.erb'
+      enableDynamicPortAllocation: !ruby/object:Overrides::Terraform::PropertyOverride
+        default_from_api: true
     custom_code: !ruby/object:Provider::Terraform::CustomCode
       constants: 'templates/terraform/constants/router_nat.go.erb'
       resource_definition: 'templates/terraform/resource_definition/router_nat.go.erb'