Chore: fix php jsonSerialize warning for currency switcher setting

[jonwaldstein]

Description

This adds the ReturnTypeWillChange flag to supress this warning:

PHP Deprecated:  Return type of Give\Framework\FieldsAPI\Properties\DonationForm\CurrencySwitcherSetting::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /Users/jonwaldstein/Herd/givewp/wp-content/plugins/give/src/Framework/FieldsAPI/Properties/DonationForm/CurrencySwitcherSetting.php on line 47

Affects

php 8.1 compatibility with the CurrencySwitcherSetting class

Visuals

N/A

Testing Instructions

Make sure the warning does not show up when interacting with the form & currency switcher.

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Relevant @unreleased tags included in DocBlocks
• Includes unit tests
• Reviewed by the designer (if follows a design)
• Self Review of code and UX completed