Bugfix: honor upstream default for enableDynamicPortAllocation

[modular-magician]

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 #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.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
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• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: fix default handling for `enableDynamicPortAllocation` to be managed by the api

Derived from GoogleCloudPlatform/magic-modules#6125