Fix race condition when flagging site as unsupported for application passwords

[itsmeichigo]

Closes WOOMOB-1450

Description

There are crash reports regarding the removal of retried requests when flagging sites as unsupported for application passwords. These are caused by the race condition when the list of retried requests is altered simultaneously from different threads.

This PR fixes the crash by ensuring that the retried request list is updated atomically with a barrier block.

Testing steps

It's not straightforward to reproduce race conditions so it's tricky to test the crash. Simply smoke testing the flow should be sufficient.

Use the test plan in pe5sF9-4Am-p2 - run TC4. Confirm that the app doesn't crash.

Testing information

• Added a unit test to confirm the race condition fix. The test fails before the fix.
• Ran TC4 with simulator iPhone 17 and confirmed the app doesn't crash.

Screenshots

N/A

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS Prototype by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS Prototype
	Build Number	pr16223-54b98ac
	Version	23.4
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	54b98ac
	Installation URL	5tmrktlo6q458

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[itsmeichigo]

Closing to target 23.4 instead.