alternatecalendar: use month names from `icu::SimpleDateFormat`
No need to maintain a list of month names as libicu provides them.
Translated month names are available in https://github.com/unicode-org/icu/tree/main/icu4c/source/data/locales
No need to maintain a list of month names as libicu provides them.
Translated month names are available in https://github.com/unicode-org/icu/tree/main/icu4c/source/data/locales