backend/s3: Update S3 client to AWS SDK Go V2

[jar-b]

This change upgrades the S3 backend AWS clients to AWS SDK Go V2. In addition to moving the backend off the aging V1 SDK, this will enable parity with the AWS provider configuration arguments and resolution of several feature requests captured in the S3 backend modernization meta-issue, #33687. Some of these enhancements have already been incorporated into the feature branch, including:

• Support for role assumption with web identities (backend/s3: Support assume_role_with_web_identity #31244)
• Support for multiple configuration and credentials files (backend/s3: Add multiple credentials and config files parameters #30493)

The remaining items captured in the meta-issue will be incorporated as standalone pull requests to avoid stretching the scope of this PR beyond its current size.

% TF_ACC=1 go test -count=1 ./internal/backend/remote-state/s3/...
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 106.875s

Closes #30443
Closes #31244
Closes #30493

Target Release

1.6.0

Draft CHANGELOG entry

UPGRADE NOTES

• Adds an assume_role_with_web_identity argument. (#31244)
• Supports the default AWS environment variables for credentials and configuration files: AWS_CONFIG_FILE and AWS_SHARED_CREDENTIALS_FILE. (#30493)
• Adds shared_config_files and shared_credentials_files arguments.
  This deprecates the shared_credentials_file argument. (#30493)
• Upgrades the S3 backend to use AWS SDK Go V2. (#30443)

[jar-b]

aws-sdk-go-base/v2 uses the latest version of golang.org/x/exp, which includes a breaking change to the slices.SortFunc signature:

func SortFunc[S ~[]E, E any](x S, cmp func(a, b E) int)

SortFunc sorts the slice x in ascending order as determined by the cmp function. This sort is not guaranteed to be stable. cmp(a, b) should return a negative number when a < b, a positive number when a > b and zero when a == b.

To continue sorting in descending order the arguments were switched (b - a). A simple sanity is included below, for reference.

// Old signature
func main() {
	sl := []int{5, 1, 3, 4, 2, 6}
	fmt.Printf("before: %v\n", sl)
	// before: [5 1 3 4 2 6]

	slices.SortFunc(sl, func(a, b int) bool {
		return a > b
	})
	fmt.Printf("after: %v\n", sl)
	// after: [6 5 4 3 2 1]
}

// New signature
func main() {
	sl := []int{5, 1, 3, 4, 2, 6}
	fmt.Printf("before: %v\n", sl)
	// before: [5 1 3 4 2 6]

	slices.SortFunc(sl, func(a, b int) int {
		return b - a
	})
	fmt.Printf("after: %v\n", sl)
	// after: [6 5 4 3 2 1]
}

Lastly, this appears to be the only place the x/exp package is utilized outside of the S3 backend, so there should be no impact elsewhere.

% rg "golang.org/x/exp" -g '!/go.*'
internal/command/test.go
17:     "golang.org/x/exp/slices"

internal/backend/remote-state/s3/backend_test.go
34:     "golang.org/x/exp/maps"

[ewbankkit]

LGTM 🚀.

% TF_ACC=1 go test -count=1 ./internal/backend/remote-state/s3/...
ok  	github.com/hashicorp/terraform/internal/backend/remote-state/s3	112.041s

[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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