Remove use of `async_trait`

[dullbananas]

Not needed anymore in rust 1.75

[phiresky]

afaik the official async trait feature uses raw futures and not boxed futures like async_trait so that might not work for the use cases in lemmy

[Nutomic]

The first step for this would be getting rid of async_trait in activitypub-federation. I tried that and ran into the exact errors mentioned in the async-trait readme. So I dont think we can get rid of async_trait in case of Lemmy.