Crons: Handle implementation-specific day-of-month and day-of-week interactions correctly in crontab schedules

While the `crontab` syntax is [standardized](https://www.ibm.com/docs/en/db2-as-a-service?topic=task-unix-cron-format), the way of handling interactions between the day-of-month and day-of-week fields varies between commonly used implementations. Take for example the schedule `0 0 1 * 1`. The first `1` indicates that this schedule should run on the first day of the month, and the second `1` indicates that the schedule should run on Monday. The way Sentry would interpret this schedule, which is also the way most UNIX systems would interpret this schedule, is that the task should run the first day of every month, and also every Monday (a union of the day-of-month and day-of-week fields). However, other systems, [such as Celery](https://docs.celeryq.dev/en/stable/reference/celery.schedules.html#celery-schedules), only run when **all** of the fields are matched (an intersection), so Celery would run this schedule only on the first day of the month, and only if that day is a Monday (way less often than Sentry would expect). Sentry itself also appears to handle certain schedules inconsistently. Take for instance [this example Cron monitor](https://sentry-sdks.sentry.io/issues/alerts/rules/crons/python/test2/details/?project=4505239425777664&statsPeriod=1h) with a schedule of `0 0 */15 * 1`. The day of month `*/15` indicates that the schedule should run every 15th day of in a month, and the `1` indicates the schedule should run every Monday. The human-readable schedule written on the page indicates that the job should run "At 00:00, every 15 days, and on Monday (UTC)"; however, if we check the next expected execution, which is three weeks from today (May 23), we see that the next expected execution is actually the next 15th day of the month, which is also a Monday (June 16). https://github.com/user-attachments/assets/63d4b329-a624-4361-a449-fc5c635b1788 We need to fix these inconsistencies within the UI itself, and also enable Sentry to support correctly ingesting crontab…