Update backend.Backend's StateMgr method to return diagnostics instead of primitive errors

[SarahFrench]

This PR is stacked on #37509

This PR updates the backend.Backend interface's StateMgr method to return diagnostics instead of errors. This will allow backends to return warnings in future, instead of just errors.

I've also updated any calling code that uses any implementations of that method. This includes test code.

Target Release

N/A

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[SarahFrench]

Testing:

pg backend:

% go test github.com/hashicorp/terraform/internal/backend/remote-state/pg   
ok      github.com/hashicorp/terraform/internal/backend/remote-state/pg 0.958s

kubernetes backend:

% go test github.com/hashicorp/terraform/internal/backend/remote-state/kubernetes
ok      github.com/hashicorp/terraform/internal/backend/remote-state/kubernetes 54.540s

gcs backend

% go test github.com/hashicorp/terraform/internal/backend/remote-state/gcs
ok      github.com/hashicorp/terraform/internal/backend/remote-state/gcs        122.954s

I don't have the resources/knowledge for testing s3 and azure Working on it!

[SarahFrench]

I just rebased this PR on top of X so that the s3 test failures resulting from diagnostic comparisons are resolved. This means it's easier to see what failures could be caused by this PR's changes.

I ran the s3 tests locally and they passed:

% go test github.com/hashicorp/terraform/internal/backend/remote-state/s3
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 12.926s

[SarahFrench]

I ran the azure tests on this PR and found the outcome was just like in this comment on another PR: #37502

• TestAccBackendAzureADAuthBasic fails due to missing permissions, I think the ability to interact with Active Directory?
• Flakey tests: The tests below fail when run alongside other tests, but pass when run individually. I think there's manipulation of ENVs that impacts across tests somewhere, but I don't want to dig into that.

--- FAIL: TestRemoteClientAccessKeyBasic (0.00s)
helpers_test.go:159: Missing ARM_SUBSCRIPTION_ID
--- FAIL: TestRemoteClientSasTokenBasic (0.00s)
helpers_test.go:159: Missing ARM_SUBSCRIPTION_ID
--- FAIL: TestAccBackendServicePrincipalLocked (0.00s)
helpers_test.go:159: Missing ARM_SUBSCRIPTION_ID

[SarahFrench]

Note for reviewers: I've run tests on the following backends:

• gcs, kubernetes, pg: Update backend.Backend's StateMgr method to return diagnostics instead of primitive errors #37496 (comment)
• s3: Update backend.Backend's StateMgr method to return diagnostics instead of primitive errors #37496 (comment)
• azure: Update backend.Backend's StateMgr method to return diagnostics instead of primitive errors #37496 (comment)

[github-actions]

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