gsoc2023 examples are growing stale

I ran cargo upgrades and the output included this:

download-manager: /volatile/rustcargo/Rustup/Arti/arti/examples/gsoc2023/download-manager/Cargo.toml
        hyper matches 0.14.28;  latest is 1.3.1

obfs4-checker: /volatile/rustcargo/Rustup/Arti/arti/examples/gsoc2023/obfs4-checker/Cargo.toml
        axum matches 0.6.20;    latest is 0.7.5

We should do one of the following:

• a. Retire these examples
• b. Decide we're going to maintain them: move them to a different location, review them, and update them to use recent hyper and axum

See also #717, which asks for something which might be rather like examples/gsoc2023/obfs4-checker.