fix: Marks `project.region_usage_restrictions` as OptionalComputed to avoid provider upgrade errors

[EspenAlbert]

Description

• Marks project.region_usage_restrictions as OptionalComputed to avoid provider upgrade errors
• Enables TestAccGovProject_withProjectOwner in CI
• Adds TestMigGovProject_regionUsageRestrictionsDefault (also enabled in the CI, error reproduction)

Link to any related issue(s): #2263 and CLOUDP-248517

Marking the attribute as Computed as earlier versions of the resource will have an empty state for this attribute. Since the state is empty and the default server value depends on cloud-gov or not:

• If cloud-gov, default value is: COMMERCIAL_FEDRAMP_REGIONS_ONLY
• else, default value is null

Example of error and solution:

mongodbatlas_project.project: Modifying... [id=664c6820c4391e7c9f58377c]
╷
│ Error: Provider produced inconsistent result after apply
│
│ When applying changes to mongodbatlas_project.project, provider "provider[\"registry.terraform.io/mongodb/mongodbatlas\"]" produced an unexpected new value: .region_usage_restrictions: was null, but now cty.StringVal("COMMERCIAL_FEDRAMP_REGIONS_ONLY").
│
│ This is a bug in the provider, which should be reported in the provider's own issue tracker.
╵

After adding Computed: true

mongodbatlas_project.project: Refreshing state... [id=664c6820c4391e7c9f58377c]

No changes. Your infrastructure matches the configuration.

Terraform has compared your real infrastructure against your configuration and found no differences, so no changes are needed.

Type of change:

• [ ] Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• [ ] New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• [ ] This change requires a documentation update
• [ ] Documentation fix/enhancement

Required Checklist:

• [x] I have signed the MongoDB CLA
• [x] I have read the contributing guides
• [x] I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• [x] I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• [x] I have added any necessary documentation (if appropriate)
• [x] I have run make fmt and formatted my code
• [x] If changes include deprecations or removals, I defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.
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Further comments

[marcosuma]

do we have an acceptance test for this?

[EspenAlbert]

@marcosuma I have been investigating a bit, and we don't have ANY test running in mongodbgov. I am looking at ways to add those tests

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[marcosuma]

Nice one!