encoding/json: add "json: " prefix to SyntaxError messages

The other named errors - UnmarshalTypeError, etc - in this package do
the same, so we should prepend the package prefix to error messages
for consistency.

Add a note to the release docs in case this is interpreted as
a breaking change.

Fixes #36221.

Change-Id: Ie24b532bbf9812e108c259fa377e2a6b64319ed4
Reviewed-on: https://go-review.googlesource.com/c/go/+/263619


Run-TryBot: Kevin Burke <kev@inburke.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Kevin Burke <kev@inburke.com>
Trust: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>