Replace old actix-web with axum

[Theodus]

Most of the work has been done: theodus/axum. But there's a recurring compiler bug preventing progress, where holding graphql_parser::query::Text across awaits can suddenly become impossible. So until it gets fixed or we figure out a workaround, we're stuck on an old version of actix-web.

More discussion on the relevant compiler bug:

• Async function leads to a "more general type" error rust-lang/rust#71723
• https://users.rust-lang.org/t/implementation-of-fnonce-is-not-general-enough/78006
• https://users.rust-lang.org/t/implementation-of-fnonce-is-not-general-enough-with-async-block/83427