go1.26: replace errors.New with fmt.Errorf

[aarongable]

It used to be the case that errors.New was more efficient than fmt.Errorf, and therefore folks said that you should use errors.New any time you were creating an error and didn't need any formatting capabilities. That is no longer the case, so we should switch to using fmt.Errorf everywhere for the sake of consistency and lower mental overhead.

Read more: https://antonz.org/go-1-26/#fmt-errorf, https://go-review.googlesource.com/c/go/+/708836