Declare STPPaymentConfiguration shared property with let to prevent swift concurrency issue

[rbenna]

Summary

Declare STPPaymentConfiguration shared property with let to prevent swift concurrency error when enabling complete mode for strict concurrency checking in the project that uses this library.

Motivation

Here's the error raised by the compiler on the declaration side @objc(sharedConfiguration) public static var shared = STPPaymentConfiguration():

Class property 'shared' is not concurrency-safe because it is non-isolated global shared mutable state; this is an error in Swift 6.

and here's the error on the use side of STPPaymentConfiguration.shared:

Reference to class property 'shared' is not concurrency-safe because it involves shared mutable state; this is an error in Swift 6

Unfortunately, import Stripe using @preconcurrency import doesn't fix the issue.