Backport of write-only attributes: internal providers should set write-only attributes to null into v1.12

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Backport

This PR is auto-generated from #36824 to be assessed for backporting due to the inclusion of the label 1.12-backport.

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

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We have several internal providers that wrap external providers and override functionality. These providers must handle setting write-only attributes to null automatically as Terraform expects this.

The mock provider in the testing framework, the unknown provider in stacks, and the override_resource functionality in the testing framework are all fixed in this PR.

Fixes #36819

Target release: v1.11.4

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Overview of commits

• 343ed32

• 0fd22e0

• 77c1cfa

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Changelog Warning

Currently this PR would target a v1.12 release. Please add a changelog entry for in the .changes/v1.12 folder, or discuss which release you'd like to target with your reviewer. If you believe this change does not need a changelog entry, please add the 'no-changelog-needed' label.

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