#62190 [13.x] Convert the down --retry datetime to GMT

#58509 added datetime support to down --retry, but the Retry-After header is formatted with DateTimeInterface::RFC7231, which appends a literal "GMT" without converting the time. In any app not running in UTC the header is off by the timezone offset:

APP_TIMEZONE=Europe/Berlin

php artisan down --retry="2026-12-31 15:30:00"

Retry-After: Thu, 31 Dec 2026 15:30:00 GMT   // should be 14:30:00 GMT

This uses Carbon's toRfc7231String(), which converts to GMT first.