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/availability-recovery/src/lib.rs

@@ -836,7 +836,7 @@ async fn handle_signal(state: &mut State, signal: OverseerSignal) -> SubsystemRe
 		OverseerSignal::Conclude => Ok(true),
 		OverseerSignal::ActiveLeaves(ActiveLeavesUpdate { activated, .. }) => {
 			// if activated is non-empty, set state.live_block to the highest block in `activated`
-			for activated in activated {
+			if let Some(activated) = activated {
 				if activated.number > state.live_block.0 {
 					state.live_block = (activated.number, activated.hash)
 				}

polkadot/node/network/bitfield-distribution/src/lib.rs

@@ -233,7 +233,7 @@ impl BitfieldDistribution {
 				})) => {
 					let _timer = self.metrics.time_active_leaves_update();
 
-					for activated in activated {
+					if let Some(activated) = activated {
 						let relay_parent = activated.hash;
 
 						gum::trace!(target: LOG_TARGET, ?relay_parent, "activated");

polkadot/node/network/bridge/src/rx/mod.rs

@@ -566,7 +566,7 @@ where
 					num_deactivated = %deactivated.len(),
 				);
 
-				for activated in activated {
+				if let Some(activated) = activated {
 					let pos = live_heads
 						.binary_search_by(|probe| probe.number.cmp(&activated.number).reverse())
 						.unwrap_or_else(|i| i);

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!(