Drop details support in cloud error ResponseError

[mstoykov]

Why?

Currently the parsing of error from the cloud api has some complicated code

k6/cloudapi/errors.go

Lines 47 to 62 in b5a6feb

	// `e.Details` is the old API version
	// TODO: do not handle `details` when the old API becomes obsolete
	var details []string //nolint:prealloc
	var detail string
	for k, v := range e.Details {
	detail = k + ": " + strings.Join(v, ", ")
	details = append(details, detail)
	}
	for k, v := range e.FieldErrors {
	detail = k + ": " + strings.Join(v, ", ")
	// atm: `details` and `field_errors` could be duplicated
	if !contains(details, detail) {
	details = append(details, detail)
	}
	}

Almost all of which is around parsing details which was teh old way the error was returned but has been deprecated and not needed for 5 years.

What?

We can probably just drop most of the code but some testing is needed.