Fixes "for loop over an Option" warnings (#6291)

Was seeing these warnings when running `cargo check --all`:

```
warning: for loop over an `Option`. This is more readably written as an `if let` statement
    --> node/core/approval-voting/src/lib.rs:1147:21
     |
1147 |             for activated in update.activated {
     |                              ^^^^^^^^^^^^^^^^
     |
     = note: `#[warn(for_loops_over_fallibles)]` on by default
help: to check pattern in a loop use `while let`
     |
1147 |             while let Some(activated) = update.activated {
     |             ~~~~~~~~~~~~~~~         ~~~
help: consider using `if let` to clear intent
     |
1147 |             if let Some(activated) = update.activated {
     |             ~~~~~~~~~~~~         ~~~
```

My guess is that `activated` used to be a SmallVec or similar, as is
`deactivated`. It was changed to an `Option`, the `for` still compiled (it's
technically correct, just weird), and the compiler didn't catch it until now.

polkadot/node/core/chain-selection/src/lib.rs

@@ -430,7 +430,7 @@ where
 						return Ok(())
 					}
 					FromOrchestra::Signal(OverseerSignal::ActiveLeaves(update)) => {
-						for leaf in update.activated {
+						if let Some(leaf) = update.activated {
 							let write_ops = handle_active_leaf(
 								ctx.sender(),
 								&*backend,