replace harshicorp/multierror with errors.Join

[djdongjin]

As titled. The replacement is done by:

1. a multierror error (allErr) -> var errs []error
2. multierror.Append(allErr, err) -> errs = append(errs, err)
3. return allErr -> return errors.Join(errs)

And if errs is empty slice or if every error in errs are nil, return errors.Join(errs) will also return a nil error.

[ktock]

Thanks