opt-out multi-threaded feature flag (#9269)

# Objective

Fixes #9113

## Solution

disable `multi-threaded` default feature

## Migration Guide
The `multi-threaded` feature in `bevy_ecs` and `bevy_tasks` is no longer
enabled by default. However, this remains a default feature for the
umbrella `bevy` crate. If you depend on `bevy_ecs` or `bevy_tasks`
directly, you should consider enabling this to allow systems to run in
parallel.

crates/bevy_ecs/Cargo.toml

@@ -12,7 +12,7 @@ categories = ["game-engines", "data-structures"]
 [features]
 trace = []
 multi-threaded = ["bevy_tasks/multi-threaded"]
-default = ["bevy_reflect", "multi-threaded"]
+default = ["bevy_reflect"]
 
 [dependencies]
 bevy_ptr = { path = "../bevy_ptr", version = "0.12.0-dev" }

crates/bevy_tasks/Cargo.toml

@@ -10,7 +10,6 @@ keywords = ["bevy"]
 
 [features]
 multi-threaded = []
-default = ["multi-threaded"]
 
 [dependencies]
 futures-lite = "1.4.0"

crates/bevy_tasks/src/lib.rs

@@ -23,9 +23,9 @@ mod usages;
 pub use usages::tick_global_task_pools_on_main_thread;
 pub use usages::{AsyncComputeTaskPool, ComputeTaskPool, IoTaskPool};
 
-#[cfg(not(target_arch = "wasm32"))]
+#[cfg(all(not(target_arch = "wasm32"), feature = "multi-threaded"))]
 mod thread_executor;
-#[cfg(not(target_arch = "wasm32"))]
+#[cfg(all(not(target_arch = "wasm32"), feature = "multi-threaded"))]
 pub use thread_executor::{ThreadExecutor, ThreadExecutorTicker};
 
 mod iter;

crates/bevy_tasks/src/single_threaded_task_pool.rs

@@ -1,4 +1,3 @@
-#[cfg(target_arch = "wasm32")]
 use std::sync::Arc;
 use std::{cell::RefCell, future::Future, marker::PhantomData, mem, rc::Rc};