[v0.11] backend/s3: Switch from github.com/terraform-providers/terraform-provider-aws to github.com/hashicorp/aws-sdk-go-base

[bflad]

Closes #20338
Closes #19992
Reference: #20374

This pull request is provided to simplify Terraform Provider migrations to Go Modules with v0.11.

Backports the S3 backend code from master to v0.11 relating to the AWS Go SDK, which removes the circular dependency with the Terraform AWS Provider and includes the addition of retry logic to the backend to prevent temporary networking or AWS service issues from failing unnecessarily.

Updated via code change then:

go mod tidy
go mod vendor

Output from acceptance testing:

--- PASS: TestBackend_impl (0.00s)
--- PASS: TestBackendConfig (0.39s)
--- PASS: TestBackendConfig_invalidKey (0.00s)
--- PASS: TestBackend (2.64s)
--- PASS: TestBackendLocked (9.81s)
--- PASS: TestBackendExtraPaths (3.08s)
--- PASS: TestBackendPrefixInWorkspace (1.61s)
--- PASS: TestKeyEnv (9.74s)
--- PASS: TestRemoteClient_impl (0.00s)
--- PASS: TestRemoteClient (1.68s)
--- PASS: TestRemoteClientLocks (12.21s)
--- PASS: TestForceUnlock (12.59s)
--- PASS: TestRemoteClient_clientMD5 (9.02s)
--- PASS: TestRemoteClient_stateChecksum (9.76s)

[ghost]

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