Formie: Pre-authenticated server-side template injection in Hidden fields

### Impact - Unauthenticated users could submit crafted values into Hidden fields (with Default value → Custom) that were evaluated as Twig during submission handling, which could lead to serious compromise of the Craft site (depending on template/sandbox behavior). - Sites with public Formie forms that include at least one Hidden field with that configuration. - No CP login for the reported chain. ### Patches - [2.2.20](https://github.com/verbb/formie/releases/tag/2.2.20), [3.1.24](https://github.com/verbb/formie/releases/tag/3.1.24) ### Workarounds - Temporarily remove Hidden fields from public forms or switch Hidden default away from Custom where feasible - Otherwise, upgrade to patched versions