Add validate_xcm_nesting to the ParentAsUmp and ChildParachainRouter (#4236)

This PR:
- moves `validate_xcm_nesting` from `XcmpQueue` into the `VersionedXcm`
- adds `validate_xcm_nesting` to the `ParentAsUmp`
- adds `validate_xcm_nesting` to the `ChildParachainRouter`


Based on discussion
[here](https://github.com/paritytech/polkadot-sdk/pull/4186#discussion_r1571344270)
and/or
[here](https://github.com/paritytech/polkadot-sdk/pull/4186#discussion_r1572076666)
and/or [here]()

## Question/TODO

- [x] To the
[comment](https://github.com/paritytech/polkadot-sdk/pull/4186#discussion_r1572072295)
- Why was `validate_xcm_nesting` added just to the `XcmpQueue` router
and nowhere else? What kind of problem `MAX_XCM_DECODE_DEPTH` is
solving? (see
[comment](https://github.com/paritytech/polkadot-sdk/pull/4236#discussion_r1574605191))

cumulus/primitives/utility/src/lib.rs

@@ -69,6 +69,9 @@ where
 			let price = P::price_for_delivery((), &xcm);
 			let versioned_xcm =
 				W::wrap_version(&d, xcm).map_err(|()| SendError::DestinationUnsupported)?;
+			versioned_xcm
+				.validate_xcm_nesting()
+				.map_err(|()| SendError::ExceedsMaxMessageSize)?;
 			let data = versioned_xcm.encode();
 
 			Ok((data, price))
@@ -526,6 +529,8 @@ impl<
 mod test_xcm_router {
 	use super::*;
 	use cumulus_primitives_core::UpwardMessage;
+	use frame_support::assert_ok;
+	use xcm::MAX_XCM_DECODE_DEPTH;
 
 	/// Validates [`validate`] for required Some(destination) and Some(message)
 	struct OkFixedXcmHashWithAssertingRequiredInputsSender;
@@ -621,6 +626,29 @@ mod test_xcm_router {
 			)>(dest.into(), message)
 		);
 	}
+
+	#[test]
+	fn parent_as_ump_validate_nested_xcm_works() {
+		let dest = Parent;
+
+		type Router = ParentAsUmp<(), (), ()>;
+
+		// Message that is not too deeply nested:
+		let mut good = Xcm(vec![ClearOrigin]);
+		for _ in 0..MAX_XCM_DECODE_DEPTH - 1 {
+			good = Xcm(vec![SetAppendix(good)]);
+		}
+
+		// Check that the good message is validated:
+		assert_ok!(<Router as SendXcm>::validate(&mut Some(dest.into()), &mut Some(good.clone())));
+
+		// Nesting the message one more time should reject it:
+		let bad = Xcm(vec![SetAppendix(good)]);
+		assert_eq!(
+			Err(SendError::ExceedsMaxMessageSize),
+			<Router as SendXcm>::validate(&mut Some(dest.into()), &mut Some(bad))
+		);
+	}
 }
 #[cfg(test)]
 mod test_trader {