[Feature] Complete Swift Strict Concurrency Check support

[gabe4coding]

Problem

Trying to migrate the app to the "complete" Swift Strict Concurrency check, and we have a blocker with Stripe, that declares a static var (STPPaymentConfiguration Singleton), which cannot be referenced since mutable.
@preconcurrency directive is not working.
So far, we are forced to use the "targeted" check, to silence this error on the compiler, preventing a complete migration.

Solution

Add the support to Swift Strict Concurrency check "complete" to the library.

[davidme-stripe]

Hello, thanks for filing this! I took a look, and unfortunately the STPPaymentConfiguration is just the tip of the iceberg: We have a lot of code that doesn't meet these requirements. We'll keep this issue open to track our work on this.

[rbenna]

Hi @davidme-stripe, I've already opened a PR to fix that, this small change will let us enable the complete strict concurrency check in our app.

[r-thefork]

Indeed, merging @rbenna's PR would unblock developers.