go.mod: bump github.com/pkg/sftp from 1.13.0 to 1.13.3

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Bumps [github.com/pkg/sftp](https://github.com/pkg/sftp) from 1.13.0 to 1.13.3.
- [Release notes](https://github.com/pkg/sftp/releases)
- [Commits](https://github.com/pkg/sftp/compare/v1.13.0...v1.13.3)

---
updated-dependencies:
- dependency-name: github.com/pkg/sftp
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

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