[7.x] Add "every 2 / 3 / 4 / 6 hours" methods to scheduler

[SjorsO]

This PR, just like #33379, adds more useful methods to the scheduler:

• everyTwoHours()
• everyThreeHours()
• everyFourHours()
• everySixHours()

These new methods go alongside the currently available hourly and twiceDaily methods (twiceDaily is essentially everyTwelveHours).

I believe scheduling cron jobs at these intervals is common enough that they deserve dedicated methods. You can currently only achieve the same behavior by writing a custom cron expression:

$schedule->job(SyncSomething::class)->cron('0 */2 * * *'); // once every 2 hours

// or, with this PR

$schedule->job(SyncSomething::class)->everyTwoHours();

It is easy to make mistakes when writing your own cron expressions. For example, if you by mistake write * */2 * * *, your job will be running 60 times every 2 hours.