sysutils/stackit: Update 0.67.0 => 0.68.0

While here, require Go 1.26+ after upstream go.mod change.

Changelog:
https://github.com/stackitcloud/stackit-cli/releases/tag/v0.68.0

PR: 297286
Reported by: Robert Gogolok <gogolok@gmail.com> (maintainer)
Approved by: osa, vvd (Mentors, implicit)