sysutils/kubo-go: builds fine with Go 1.26

Remove Go version pin and undeprecate.
I presume USES=go:1.25+ was originally meant.

Approved by: portmgr (build fix blanket)
MFH: 2026Q3

(cherry picked from commit 717e3028f5710aa5ecade2ab72c64941e484007e)