style: use thread-safe localtime_r

[DSpeichert]

Pull Request Prelude

• I have followed proper The Forgotten Server code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Issues addressed: Fixed not thread-safe use of localtime()

[DSpeichert]

Looks like that's a WIP because our fmt is too old.

[MillhioreBT]

Looks like that's a WIP because our fmt is too old.

I think you just need to add the#include <fmt/chrono.h>, right?