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/network/statement-distribution/src/lib.rs

@@ -2017,7 +2017,7 @@ impl<R: rand::Rng> StatementDistributionSubsystem<R> {
 					}
 				}
 
-				for activated in activated {
+				if let Some(activated) = activated {
 					let relay_parent = activated.hash;
 					let span = PerLeafSpan::new(activated.span, "statement-distribution");
 					gum::trace!(