Migrate to Observable (#33)

# Migrate to Observable

## ♻️ Current situation & Problem
This PR migrates SpeziScheduler to use `@Observable` and upgrades to the
latest Spezi dependencies.


## ⚙️ Release Notes 
* Upgrade SpeziScheduler
* Migrate to latest Spezi version
* Removes special timeZone handling for now


## 📚 Documentation
Documentation was updated where needed.

## ✅ Testing
Tests were added to ensure we remain compatible with previous Codable
representations.


## 📝 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).

CONTRIBUTORS.md

@@ -12,3 +12,4 @@ Spezi Scheduler contributors
 ====================
 
 * [Paul Schmiedmayer](https://github.com/PSchmiedmayer)
+* [Andreas Bauer](https://github.com/Supereg)

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.7
+// swift-tools-version:5.9
 
 //
 // This source file is part of the Stanford Spezi open-source project
@@ -14,14 +14,14 @@ import PackageDescription
 let package = Package(
     name: "SpeziScheduler",
     platforms: [
-        .iOS(.v16)
+        .iOS(.v17)
     ],
     products: [
         .library(name: "SpeziScheduler", targets: ["SpeziScheduler"])
     ],
     dependencies: [
-        .package(url: "https://github.com/StanfordSpezi/Spezi", .upToNextMinor(from: "0.7.0")),
-        .package(url: "https://github.com/StanfordSpezi/SpeziStorage", .upToNextMinor(from: "0.4.1"))
+        .package(url: "https://github.com/StanfordSpezi/Spezi", .upToNextMinor(from: "0.8.0")),
+        .package(url: "https://github.com/StanfordSpezi/SpeziStorage", .upToNextMinor(from: "0.5.0"))
     ],
     targets: [
         .target(