feat(checkout): CHECKOUT-10352 Swap phone LD experiment with a checkout setting

[bc-maxy]

What/Why?

Once we merge the SDK side: bigcommerce/checkout-sdk-js#3380
We can then just use a checkout setting instead of a LD flag to control rendering of new phone validation.

Since this is a controlled rollout which customers can opt-in or -out themselves, I also removed all other restrictions that we had previously in place like not rendering the component for Paypal Braintree merchants or not rendering when merchants set maxLength limits or placeholders on phone input fields.

Rollout/Rollback

Revert the PR

Testing

Updated CI tests.

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Note

Medium Risk
Changes checkout address phone input behavior for merchants who enable the setting, including PayPal Fastlane and fields with max length/placeholders; rollout is setting-controlled but affects a core checkout form path.

Overview
Replaces the LaunchDarkly experiment CHECKOUT-9019.use_new_phone_number_validation with the checkout setting isPhoneNumberValidationEnabled (default off when missing) to control the IntlTelInput-based phone field on address forms.

AddressForm no longer special-cases PayPal Fastlane custom checkout or reads providerWithCustomCheckout for phone rendering; only the new setting gates the validated phone UI.

DynamicFormFieldSelector always uses PhoneFormField for telephone fields when the setting is on, including when merchants configure maxLength or a placeholder—those cases previously forced the legacy tel input. Props are renamed from isNewPhoneValidationExperimentEnabled to isPhoneNumberValidationEnabled through the form field stack, with tests updated to match.

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[bc-maxy]

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