Backport of always allow computed-only changes in the plan into v1.2

[teamterraform]

Backport

This PR is auto-generated from #31509 to be assessed for backporting due to the inclusion of the label 1.2-backport.

The below text is copied from the body of the original PR.

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Allow the provider to plan any compatible value for computed-only attributes. We currently fail here if there's an apparent config value at this point and the provider set a new computed value.

We don't need to check the config for null here, because under normal circumstances there should never be a config value for a computed-only attribute in the first place. There however could be 2 reasons we find one at this point:

• A user set ignore_changes on a computed attribute and ignored the warning, causing us to insert the prior state value into the plan-time config. This unfortunately is only a warning to prevent breaking configs which are currently working with legacy providers.
• Core currently fails to validate if config attributes are computed, and a provider not using one of the official SDKs may not have sufficient validation itself.

Neither of these cases constitute an error in the plan created by the provider, so we can let it pass the AssertPlanValid test.

[github-actions]

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[github-actions]

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