#3457 adds the `doc_markdown` clippy lint, which checks doc comments to make sure code identifiers are escaped with backticks. This causes a lot of lint errors, so this is one of a number of PR's that will fix those lint errors one crate at a time.
This PR fixes lints in the `bevy_tasks` crate.
* Switch from the deprecated `multitask` crate to `async-executor`
* async-executor appears to be essentially multitask 0.3
* use block_on in futures_lite instead of pollster because futures_lite is already in the dependency list of async-executor
