Fix GESTIS provider

[Schallaven]

Does not work anymore due to API change (now uses JSON requests)

See also: #1 (comment) by @6543. Working solution at https://github.com/Calciumdibromid/CaBr2 (Do I need to request an authorization token from GESTIS?)

[6543]

Well kind a, there is a hardcoded one 😆

[6543]

@Schallaven
https://github.com/Calciumdibromid/CaBr2/blob/c0d65e0e45cb67ed0ee399cf89297ea7c0564865/src-tauri/cabr2_search/src/gestis/mod.rs#L34-L41