Temporal: Adjust era code of Japanese pre-Meiji era

As of the current state of the Intl Era and Month Codes proposal,
https://tc39.es/proposal-intl-era-monthcode/#sec-temporal-canonicalizeeraincalendar
the canonical name of this era is "japanese".

See: tc39/proposal-temporal#2901

test/staging/Intl402/Temporal/old/japanese-before-era.js

@@ -59,5 +59,5 @@ date = Temporal.PlainDate.from({
   calendar: "japanese"
 });
 assert.sameValue(`${ date }`, "1868-01-01[u-ca=japanese]");
-assert.sameValue(date.era, "ce");
+assert.sameValue(date.era, "japanese");
 assert.sameValue(date.eraYear, 1868);