Use go 1.26 for upgrade workflow

[mattlord]

Otherwise it fails with main / v24 as building the binaries/image fails when using anything older than 1.26.0.

You can see the last failed run here: https://github.com/planetscale/vitess-operator/actions/runs/22810042434/job/66165187951

Run go get -d vitess.io/vitess@main
go: -d flag is deprecated. -d=true is a no-op
go: downloading vitess.io/vitess v0.10.3-0.20260306221017-c5828a6672ae
go: vitess.io/vitess@main: vitess.io/vitess@v0.10.3-0.20260306221017-c5828a6672ae requires go >= 1.26.0 (running go 1.25.6; GOTOOLCHAIN=local)