encoding/json/v2: report error on time.Duration without explicit format
The default representation of a time.Duration is still undecided.
In order to keep the future open, report an error on a time.Duration
without an explicit format flag provided.
Updates #71631
Change-Id: I08248404ff6551723851417c8188a13f53c61937
Reviewed-on: https://go-review.googlesource.com/c/go/+/682455
Auto-Submit: Joseph Tsai <joetsai@digital-static.net> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Damien Neil <dneil@google.com>