deps: upgrade axum to 0.8

[taliaferro]

This PR upgrades the Axum dependency to 0.8 to take advantage of new features in axum -- chiefly, the ability to listen on anything that implements axum::serve::Listener, instead of only TCP ports. This makes it possible to bind the server to a local Unix domain socket, like so:

// set up server as shown in example code
let socket_path = "/var/run/myapp/server.sock";
let socket_listener = tokio::net::UnixListener::bind(&socket_path)?;
server.serve_listener(socket_listener).await?;

[decathorpe]

Left two quick comments for things I noticed immediately. Will take another look tomorrow.

[decathorpe]

Thanks, the changes look good to me now. However, axum 0.8 seems to have bumped its own MSRV to 1.75, which is newer than the currently documented one of 1.70. Raising it is not a problem, I'll do that, rebase your PR, and re-run the CI ... hopefully all should be green then.

[decathorpe]

I pushed changes to raise the MSRV to 1.75, this should unblock the axum update here. But it looks like I can't rebase your PR myself.

[Hate9]

@taliaferro I'd love to see this merged - what else is still needed to rebase and merge this?

[decathorpe]

@Hate9 I was waiting for this PR to be rebased against current git main to pick up the fixed / updated CI settings.

But given that this has been sitting around for a few months without that happening, I'll make the axum v0.8 changes myself.

Note that this will only land in dxr v0.8.0 (i.e. the next major release) since axum is part of the public API, and I still want to make some breaking changes before I publish 0.8.0, so it will not happen immediately :)