encoding/json: json.UnsupportedValueError returns an Invalid Value instead of a Float value [1.27 backport]

[gopherbot]

@dsnet requested issue #81176 to be considered for backport to the next 1.27 minor release.

@gopherbot Please backport to 1.27. This is a regression in encoding/json causing it to no longer populate the UnsupportedValueError.Value field.