cache/s3: only request required upload checksums

[crazy-max]

fixes #6969

This updates the vendored AWS SDK for Go v2 modules and fixes S3 cache exports so BuildKit no longer sends optional trailing checksum uploads that force Content-Encoding: aws-chunked.

The dependency update is needed because the old transfer manager API doesn't expose RequestChecksumCalculation. The newer transfer manager release exposes that policy directly, so BuildKit can apply the same when_required behavior at the upload layer that it already applies at the S3 client layer.

The S3 cache client now passes RequestChecksumCalculationWhenRequired to the transfer manager, matching the existing S3 client configuration and allowing the SDK to avoid optional default CRC32 upload checksums while still preserving required or explicitly requested checksums.

BuildKit already configured the AWS S3 client with RequestChecksumCalculationWhenRequired:

buildkit/cache/remotecache/s3/s3.go

Line 451 in b25689e

aws_config.WithRequestChecksumCalculation(aws.RequestChecksumCalculationWhenRequired),

But the S3 cache exporter uses the higher-level transfer manager for uploads. Older transfer manager releases defaulted an empty checksum algorithm to CRC32 internally, which made upload requests explicitly request a trailing checksum. For streamed uploads, that explicit trailing checksum forces aws-chunked request encoding, and some S3-compatible backends reject that encoding with 501 NotImplemented.