Demand newer async-channel version (bevyengine#7301)

After bevyengine#6503, bevy_render uses the `send_blocking` method introduced in async-channel 1.7, but depended only on ^1.4.
I saw this after pulling main without running cargo update.

# Objective

- Fix minimum dependency version of async-channel

## Solution

- Bump async-channel version constraint to ^1.8, which is currently the latest version.

NOTE: Both bevy_ecs and bevy_tasks also depend on async-channel but they didn't use any newer features.

crates/bevy_render/Cargo.toml

@@ -76,4 +76,4 @@ basis-universal = { version = "0.2.0", optional = true }
 encase = { version = "0.4", features = ["glam"] }
 # For wgpu profiling using tracing. Use `RUST_LOG=info` to also capture the wgpu spans.
 profiling = { version = "1", features = ["profile-with-tracing"], optional = true }
-async-channel = "1.4"
+async-channel = "1.8"