Enable clippy::manual_flatten (#8739)

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

Release Notes:

- N/A

tooling/xtask/src/main.rs

@@ -101,7 +101,6 @@ fn run_clippy(args: ClippyArgs) -> Result<()> {
         "clippy::iter_kv_map",
         "clippy::iter_overeager_cloned",
         "clippy::let_underscore_future",
-        "clippy::manual_flatten",
         "clippy::map_entry",
         "clippy::needless_arbitrary_self_type",
         "clippy::needless_borrowed_reference",