encoding/json/v2: remove issue reference in Duration formatting error
The json/v2 working group decided to commit to no default representation
for time.Duration for the foreseeable future.
Thus, we can remove the issue reference in the error message.
If JavaScript (TC39) formally adopts the Temporal.Duration type,
which uses ISO 8601 as the JSON representation for a duration,
we may consider changing the default.
Switching from no default to some default in the future
is generally a compatible change.
Updates #71631
Change-Id: I6f71a0fa97dcefba56aab0c0ddf445d9d38f6e58
Reviewed-on: https://go-review.googlesource.com/c/go/+/735080 Reviewed-by: Michael Pratt <mpratt@google.com> Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
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