Deprecate or remove FetchCargoTarball

[yzx9]

The rustPlatform.fetchCargoTarball function is going to be deprecated in 25.05. However, we are still recommending fetchCargoTarball (ranking it before importCargoLock).

To align with future changes, we should stop recommending or consider removing fetchCargoTarball, and may add fetchCargoVendor

Nixpkgs Release Notes 25.05

https://github.com/NixOS/nixpkgs/blob/4ac82c1d5c627bb70f1403e32f2efff8f7c0cfc4/doc/release-notes/rl-2505.section.md?plain=1#L32-L34

The rustPlatform.fetchCargoTarball function is deprecated, because it relied on cargo vendor not changing its output format to keep fixed-output derivation hashes the same, which is a Nix invariant, and Cargo 1.84.0 changed cargo vendor's output format. It should generally be replaced with rustPlatform.fetchCargoVendor, but rustPlatform.importCargoLock may also be appropriate in some circumstances. rustPlatform.buildRustPackage users must set useFetchCargoVendor to true and regenerate the cargoHash.