`Utc::now` instead of `SystemTime::now` for `catalyst-signed-doc` crate

[Mr-Leshiy]

Summary

• Replace SystemTime::now usage with chrono::Utc::now for consistent time handling for the catalyst-signed-doc crate.

Description

• Update all instances of std::time::SystemTime::now to use chrono::Utc::now instead.
• Reason: This ensures consistent, standardized UTC timestamps across the application. Using SystemTime::now can lead to potential time drift and synchronization issues between the backend and frontend, which can have security implications.
• Reference PR: This approach follows the precedent set in other related repositories. See discussion and implementation here: fix(rust/signed-doc): Replacing SystemTime::now with Utc::now #610