[6.x] Fix localized dates leaking into later template usages

[duncanmcclean]

This pull request fixes an issue where date modifiers would ignore localize_dates_in_modifiers => false and output dates in the display timezone — but only when the same date had already been output plainly earlier in the template.

This was happening because Carbon instances are mutable, and a dated entry's date is a single memoized instance shared by every usage in the template. The Localize middleware's toStringFormat callback called setTimezone() on it directly, so the first plain {{ date }} output shifted the shared instance into the display timezone and every later modifier saw the shifted date. (Date fieldtype values weren't affected, since augmentation creates a fresh Carbon instance on each access — which is why the behaviour seemed inconsistent between fields.)

This PR fixes it by copying the date before localizing it, both in the middleware's toStringFormat callback and in CoreModifiers::carbon(), which had the same mutation in the other direction when localize_dates_in_modifiers is enabled.

Fixes #13869