Change default cron schedules from @every 24h to @midnight

[jpraet]

The basic maintenance crons are scheduled @every 24h by default:
https://docs.gitea.io/en-us/config-cheat-sheet/#basic-cron-tasks---enabled-by-default

In practice, this means they will be triggered every 24h from the time that gitea has started. A better default for these would be to perform these tasks at midnight instead.

Fixes #16107.

⚠️ BREAKING ⚠️

The default schedule for basic cron tasks will now occur at midnight rather than at 24 hours after start-up.

At time of merging this changes the defaults for:

cron.archive_cleanup
cron.repo_health_check
cron.check_repo_stats
cron.cleanup_hook_task_table
cron.update_migration_poster_id
cron.sync_external_users

[zeripath]

We need to get better at making the first comment in a PR represent what the PR is doing to explain to future readers what the aim and intention of the PR was.

As this is technically breaking I've added the breaking label and a breaking section to the first comment.