backend/s3: prevent keys containing double slashes

[jar-b]

With the AWS Go SDK V2, URI cleaning of S3 object URLs is no longer done by default. As such, key values containing double slashes (//) could introduce situations where an existing statefile cannot be read. This additional validation step disallows keys containing double slashes to prevent this.

Prevention of leading and trailing slashes (also potential failure points with AWS Go SDK v2) is already done via the validateStringS3Path validator.

% TF_ACC=1 go test ./internal/backend/remote-state/s3/...
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 122.148

Relates #33687
Relates hashicorp/terraform-provider-aws#33358 (changes to URI cleaning discovered during SDK upgrade of this resource in the AWS Provider)

Target Release

1.6.x

Draft CHANGELOG entry

N/A - validation change only.

[gdavison]

Looks good! 🚀

[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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