Enable clippy::useless_conversion (#8767)

This PR enables the
[`clippy::useless_conversion`](https://rust-lang.github.io/rust-clippy/master/index.html#/useless_conversion)
rule and fixes the outstanding violations.

Release Notes:

- N/A

tooling/xtask/src/main.rs

@@ -111,7 +111,6 @@ fn run_clippy(args: ClippyArgs) -> Result<()> {
         "clippy::suspicious_to_owned",
         "clippy::type_complexity",
         "clippy::unnecessary_to_owned",
-        "clippy::useless_conversion",
         "clippy::vec_init_then_push",
     ];