Update aws-c-io to fix dispatch queue leak

[xiazhvera]

Issue #, if available:

Description of changes:

1. aws-c-io -> v0.21.4
2. Update CI to test with Xcode_16.3. Github runner image changes the Xcode support, and no longer provide simulator runtime for Xcode_16.2 and below ([macOS] Xcode support policy will be changed on August 11, 2025 actions/runner-images#12541). The "proper" way might be manually download the simulator runtime so we can guarantee we always have the specific simulator, however, in that case, we would need download 22+G resources in CI. For now, I simply update to use Xcode_16.3 so we don't need to download 22G simulator runtimes.

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