Backups: Replace usage of deprecated AWS go SDK s3 features

[mattlord]

Description

This addresses static check / linter errors we've been getting about usage of deprecated AWS go SDK features related to s3:

SA1019: manager.NewUploader is deprecated: superceded by feature/s3/transfermanager. See https://github.com/aws/aws-sdk-go-v2/discussions/3306 (staticcheck)

SA1019: uploader.Upload is deprecated: superceded by feature/s3/transfermanager. See https://github.com/aws/aws-sdk-go-v2/discussions/3306 (staticcheck)

SA1019: manager.Uploader is deprecated: superceded by feature/s3/transfermanager. See https://github.com/aws/aws-sdk-go-v2/discussions/3306 (staticcheck)

SA1019: aws.EndpointResolverWithOptionsFunc is deprecated: The global endpoint resolution interface is deprecated. See deprecation docs on [EndpointResolver]. (staticcheck)

SA1019: aws.Endpoint is deprecated: This structure was used with the global [EndpointResolver] interface, which has been deprecated in favor of service-specific endpoint resolution. See the deprecation docs on that interface for more information. (staticcheck)

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

AI Disclosure

My boy Claude did the heavy lifting here.

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.70%. Comparing base (f6d3ce2) to head (0723e5a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19391      +/-   ##
==========================================
+ Coverage   69.61%   69.70%   +0.09%     
==========================================
  Files        1614     1614              
  Lines      216320   216316       -4     
==========================================
+ Hits       150593   150789     +196     
+ Misses      65727    65527     -200     

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