azuread_group_role_management_policy - stop drift by making approval_status computed

[wyattfry]

This PR makes the approval_status consistent with the other properties in this resource that are Computed Optional, and resolves a bug where, when an activation_rules block is configured, previously an impossible change would always appear in plans.

I added a test (TestGroupRoleManagementPolicy_activationRules) that fails when run against the main branch:

   testcase.go:122: Step 1/2 error: After applying this test step, the non-refresh plan was not empty.
        stdout:
        
        
        Terraform used the selected providers to generate the following execution
        plan. Resource actions are indicated with the following symbols:
          ~ update in-place
        
        Terraform will perform the following actions:
        
          # azuread_group_role_management_policy.test will be updated in-place
          ~ resource "azuread_group_role_management_policy" "test" {
                id           = "Group_9dff2f65-d602-45ca-8195-55f0cccb7a1a_f305ca62-d9ae-44c7-b8e4-4b4ca5198e84"
                # (4 unchanged attributes hidden)
        
              ~ activation_rules {
                    # (6 unchanged attributes hidden)
        
                  - approval_stage {
                    }
                }
        
                # (3 unchanged blocks hidden)
            }
        
        Plan: 0 to add, 1 to change, 0 to destroy.
2025-03-14T09:05:35.383-0400 [WARN]  sdk.helper_schema: Previously configured provider being re-configured. This can cause issues in concurrent testing if the configurations are not equal.: tf_rpc=Configure tf_req_id=9bef211b-7ce6-72c0-0c43-32c0e8c4d9e1 tf_provider_addr=registry.terraform.io/hashicorp/azuread
--- FAIL: TestGroupRoleManagementPolicy_activationRules (52.86s)

FAIL

Fixes #1398

Test:

[mbfrahry]

LGTM! I'd just leave a note in the description that you're matching the consistency of the rest of the blocks in this resource

[wyattfry]

LGTM! I'd just leave a note in the description that you're matching the consistency of the rest of the blocks in this resource

Certainly, I've updated it now.