Once minimum Rust version is 1.76 remove use of async_trait

[abizjak]

Currently in a few places we define traits with async methods.

In the past it was necessary to use the async_trait macro for this, which is not very ergonomic since when a user wants to implement the trait they can't use rust-analyzer autocompletion to "implement missing members" easily. It generates the wrong placeholders.

In Rust 1.76 there is sufficient support for async methods in traits so once we set MSRV to that we should revise the use of async_trait macro.