ref: Remove actix-web in favor of axum (NATIVE-193) (#544)

Replaces the usage of `actix-web` in favor of `axum`, and thus removes our last dependency on tokio < 1 and reduces the number of thread pools that we use back to one for web/io tasks, and one for cpu-intensive tasks.
It also removes some compatibility code that was needed previously to run on tokio < 1.

Co-authored-by: Floris Bruynooghe <flub@sentry.io>

CHANGELOG.md

@@ -25,6 +25,7 @@
 - Introduced the `max_concurrent_requests` config setting, which limits the number of requests that will be processed concurrently. It defaults to `Some(120)`. ([#521](https://github.com/getsentry/symbolicator/pull/521), [#537](https://github.com/getsentry/symbolicator/pull/537))
 - Symbolication tasks are being spawned on a tokio 1 Runtime now. ([#531](https://github.com/getsentry/symbolicator/pull/531))
 - Symbolicator now allows explicitly versioning its caches. It will fall back to supported outdated cache versions immediately instead of eagerly waiting for updated cache files. Recomputations of newer cache versions are being done lazily in the background, bounded by new settings `max_lazy_redownloads` and `max_lazy_recomputations` for downloaded and derived caches respectively. ([#524](https://github.com/getsentry/symbolicator/pull/524), [#533](https://github.com/getsentry/symbolicator/pull/533), [#535](https://github.com/getsentry/symbolicator/pull/535))
+- Remove actix-web in favor of axum. This changes the web framework and also completely switches to the tokio 1 Runtime. ([#544](https://github.com/getsentry/symbolicator/pull/544))
 
 ### Fixes