`timestamp_now()` is defined in `validator_monitor` but used as a general-purpose utility

[jimmygchen]

Raised during review of #9024.

Description

timestamp_now() from validator_monitor is used as a general-purpose time utility across multiple modules (e.g., data_column_verification, data_availability_checker). This has two problems:

1. Poor encapsulation — it's a generic time utility living in validator_monitor, which is confusing when imported elsewhere.
2. Slot clock incompatibility — timestamp_now() calls SystemTime::now() directly, bypassing the slot clock. This can lead to inconsistent behavior in tests.

[michaelsproul]

Yeah it should just go in the bin I think, in favour of SlockClock::duration_now