Fix the gke script to work with older versions of kubectl

Fixes an issue caused by !1471 (merged)

where users of older kubectl versions, like 1.16 (which is still decently recent) would get this error:

Error: invalid argument "client" for "--dry-run" flag: strconv.ParseBool: parsing "client": invalid syntax

This is because the client option to dry-run wasn't added until kubectl 1.18