make enableProxyProtocol field updatable in service_attachment

[modular-magician]

Fixes: hashicorp/terraform-provider-google#15705

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• 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).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
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Release Note Template for Downstream PRs (will be copied)

compute: added update support for `enable_proxy_protocol` to`google_compute_service_attachment` resource

Derived from GoogleCloudPlatform/magic-modules#8840