backend/s3: add use_legacy_workflow argument to modernization branch

[jar-b]

Adds an optional use_legacy_workflow argument to preserve the authentication flow aws-sdk-go-base V1 implemented. This does not align with the flow of the AWS CLI or SDK's and will be phased out over several minor releases. In v1.6.0 this argument will default to true (ie. no user facing changes).

% TF_ACC=1 go test -count=1 ./internal/backend/remote-state/s3/...
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 103.779s

Relates #33631
Relates #30443
Relates hashicorp/aws-sdk-go-base#595

Target Release

1.6.0

[gdavison]

One comment, otherwise looks good 🚀

[jar-b]

Removed the deprecation and added one additional test case to cover an explicitly disabled legacy workflow.

% TF_ACC=1 go test -count=1 ./internal/backend/remote-state/s3/...
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 98.150s

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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