Add replacement_method to IGM and RIGM update_policy

[modular-magician]

This PR adds a replacement_method argument to the update_policy stanza of google_compute_instance_group_manager and google_compute_region_instance_group_manager resources. This allows preserving instance names during an upgrade.

Fixes hashicorp/terraform-provider-google#7417

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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).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• 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).
• 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: added `replacement_method` field to `update_policy` block of `google_compute_instance_group_manager`

compute: added `replacement_method` field to `update_policy` block of `google_compute_region_instance_group_manager`

Derived from GoogleCloudPlatform/magic-modules#4258