Support Prometheus alert rule for backup missing expected schedule

[H-M-Quang-Ngo]

Context

As the issue velero-io/velero#9559 is resolved, and now Velero exposes an "expected interval metric" for scheduled backup, we can now support a general alert rule for when a scheduled backup misses its expected schedule.
It can be done when upstream releases the latest version that includes the metric.

What needs to get done

• Add a general VeleroNoNewBackup alert rule that dynamically works for any scheduled backup. Current VeleroNoNewDailyBackup and VeleroNoNewWeeklyBackup could be removed.
• Update unit test
• Update charmhub document

Definition of Done

New alert rule is added and document is updated.

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-8510.

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