[WCiOS17] Address remaining WPAuth warnings

[iamgabrielma]

Part of, closes WOOMOB-1003

Description

This PR should be the last remaining issue from clearing all iOS17-related warnings. We update the trait observation for the remaining items from the WPAuth library:

• SiteInfoHeaderView: Used for login into self-hosted sites (LoginSelfHostedViewController, LoginUsernamePasswordViewController)
• NuxButton and its stacked variant: used all around, but mostly around login/auth flows.
• SubheadlineButton: only used for styling signupTermsButton (in the button to open our Terms & conditions)

Test Steps

As with previous ones I tested the login flow for self hosted site, and both the orientation and dark/light switches seem to work correctly, but looping Kiwi since you see those flows much more often. Let me know if anything seems off.

[dangermattic]

	1 Warning
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[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	pr16437-48b63fe
	Version	23.8
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	48b63fe
	Installation URL	5vtf8tjhmg4go

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

[RafaelKayumov]

LGTM