Backport of chore: Bump to go version 1.26.5 into release/0.21.x

[hc-github-team-secure-boundary]

Backport

This PR is auto-generated from #6770 to be assessed for backporting due to the inclusion of the label backport/0.21.x.

The below text is copied from the body of the original PR.

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Description

This PR bumps the go version to 1.26.5 in order to resolve the following

 Scanned file:{path:"to-scan.SxmA/boundary"} in 1m51.3s - found 4 result(s)
  » Go Modules Scanner
    ⚠︎ found NVD reported vulnerability CVE-2026-39822 in cpe:2.3:a:golang:go:1.26.4:*:*:*:*:*:*:*
        to-scan.SxmA/boundary:1:1
    ⚠︎ found NVD reported vulnerability CVE-2026-42505 in cpe:2.3:a:golang:go:1.26.4:*:*:*:*:*:*:*
        to-scan.SxmA/boundary:1:1
    ⚠︎ found NVD reported vulnerability CVE-2026-39822 in cpe:2.3:a:golang:go:1.26.4:*:*:*:*:*:*:*
        to-scan.SxmA/boundary:1:1
    ⚠︎ found NVD reported vulnerability CVE-2026-42505 in cpe:2.3:a:golang:go:1.26.4:*:*:*:*:*:*:*
        to-scan.SxmA/boundary:1:1

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.
• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• If applicable, I've documented the impact of any changes to security controls.
  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.

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Overview of commits

• b0b0c49