If you're looking for a simple, but not perfect solution consider using time.RFC3339 constant. But also know that there are differences between ISO8601 which are too complex for this answer.
Replacing the sign in the format with a Z triggers the ISO 8601 behavior. Which is exactly time.RFC3339. If you are wanting the string output to end in 'Z' what you need to do is convert to the UTC zone.
package main
import (
"fmt"
"time"
)
func main() {
fmt.Println(time.Now().UTC().Format("2006-01-02T15:04:05Z07:00"))
}
// this is the same format used by RFC3339. just a note on why.
ISO8601 allows for variable levels of granularity. You can have just a year, year+month, year+month+day, add a time portion, and optionally have a timezone portion. Go's built-in time parsing, however, requires you to know ahead-of-time which parts will be included.
The github.com/btubbs/datetime library provides a more flexible parser that can handle all the commonly used ISO8601 formats. See https://github.com/btubbs/datetime