Minor concurrency adjustments (#114)

# Minor concurrency adjustments

## ♻️ Current situation & Problem
With Xcode Beta 5 there are new region-based concurrency checks that
trigger a warning about strongly transferred parameters (see
[SE-0414](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0414-region-based-isolation.md#using-transferring-to-simplify-nonisolated-actor-initializers-and-actor-deinitializers))
when passing actor isolated closures that is not explicitly marked as
`@Sendable`.
This PR resolves those warning by adding an explicit `@Sendable`
annotation.

Interestingly the Swift Package Index marks Spezi 1.7.1 to be Swift 6
compatible even though they compile the project with Beta 5.

## ⚙️ Release Notes 
* Minor concurrency adjustments.


## 📚 Documentation
--


## ✅ Testing
--

## 📝 Code of Conduct & Contributing Guidelines 

By submitting creating this pull request, you agree to follow our [Code
of
Conduct](https://github.com/StanfordSpezi/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordSpezi/.github/blob/main/CONTRIBUTING.md):
- [x] I agree to follow the [Code of
Conduct](https://github.com/StanfordSpezi/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordSpezi/.github/blob/main/CONTRIBUTING.md).

Sources/Spezi/Spezi/SpeziAppDelegate.swift

@@ -166,7 +166,7 @@ open class SpeziAppDelegate: NSObject, ApplicationDelegate, Sendable {
 
         let result: Set<BackgroundFetchResult> = await withTaskGroup(of: BackgroundFetchResult.self) { @MainActor group in
             for handler in handlers {
-                group.addTask { @MainActor in
+                group.addTask { @Sendable @MainActor in
                     await handler.receiveRemoteNotification(userInfo)
                 }
             }

Sources/Spezi/Spezi/SpeziNotificationCenterDelegate.swift

@@ -29,7 +29,7 @@ class SpeziNotificationCenterDelegate: NSObject {
             }
 
             for handler in delegate.spezi.notificationHandler {
-                group.addTask { @MainActor in
+                group.addTask { @Sendable @MainActor in
                     await handler.handleNotificationAction(response)
                 }
             }