run go fix ./..., add as step in bk

[michel-laterman]

What is the problem this PR solves?

Modernizes code by running go fix ./... with go v1.26
Add a check:fix target in the magefile, and add that as a target in check:ci that runs as a part of the buildkite pipeline

[michel-laterman]

Keeping this as a draft until we update to go v1.26

[mergify]

This pull request does not have a backport label. Could you fix it @michel-laterman? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.