bump serialization version (#207)

### What changes are you making?

Bumps the Android library’s `kotlinx-serialization-json` dependency from `1.6.3` to `1.7.3`.

`1.7.3` is the latest serialization runtime line that matches our current compatibility target: the Android SDK is compiled with a newer Kotlin 2.x compiler, but its published Kotlin `apiVersion` /
`languageVersion` remain pinned to Kotlin 2.0 for consumer compatibility. Newer kotlinx serialization releases move onto newer Kotlin baselines, so this keeps the runtime reasonably current without raising the
Kotlin consumer floor.

Also updates Dependabot rules to allow `1.7.x` updates while continuing to block `1.8+` until we intentionally raise the Kotlin API/language floor.

## Verification

- `./gradlew :lib:test :lib:apiCheck`
- `git -c core.fsmonitor=false diff --check`

<!-- Please describe why you are making these changes -->

### How to test

<!-- Please outline the steps to test your changes -->

---

### Before you merge

> [!IMPORTANT]
> - [ ] I've added tests to support my implementation
> - [x] I have read and agree with the [Contribution Guidelines](./CONTRIBUTING.md)
> - [x] I have read and agree with the [Code of Conduct](./CODE_OF_CONDUCT.md)
> - [ ] I've updated the relevant platform README (`platforms/swift/README.md` and/or `platforms/android/README.md`)

---

<details>
<summary>Releasing a new Swift version?</summary>

- [ ] I have bumped the version in `ShopifyCheckoutKit.podspec`
- [ ] I have bumped the version in `platforms/swift/Sources/ShopifyCheckoutKit/ShopifyCheckoutKit.swift`
- [ ] I have updated `platforms/swift/CHANGELOG.md`
- [ ] I have updated the SwiftPM/CocoaPods version snippets in `platforms/swift/README.md` (major version only)

</details>

<details>
<summary>Releasing a new Android version?</summary>

- [ ] I have bumped the `versionName` in `platforms/android/lib/build.gradle`
- [ ] I have updated `platforms/android/CHANGELOG.md`
- [ ] I have updated the Gradle/Maven version snippets in `platforms/android/README.md`

</details>

> [!TIP]
> See the [Contributing documentation](./CONTRIBUTING.md) for the full release process per platform.

Author: kiftio

.github/dependabot.yml

@@ -26,9 +26,10 @@ updates:
       # Keep Kotlin on 1.9.x for compatibility
       - dependency-name: "org.jetbrains.kotlin.*"
         versions: [">=2.0.0"]
-      # Keep kotlinx-serialization on 1.6.x (compatible with Kotlin 1.9.x)
+      # Keep kotlinx-serialization on 1.7.x while the library's Kotlin API/language
+      # consumer floor is pinned to 2.0.
       - dependency-name: "org.jetbrains.kotlinx:kotlinx-serialization-json"
-        versions: [">=1.7.0"]
+        versions: [">=1.8.0"]
       # Keep Mockito on 4.x (requires JVM 1.8)
       - dependency-name: "org.mockito*:*"
         versions: [">=5.0.0"]
@@ -50,9 +51,10 @@ updates:
       # Keep Kotlin on 1.9.x for compatibility
       - dependency-name: "org.jetbrains.kotlin.*"
         versions: [">=2.0.0"]
-      # Keep kotlinx-serialization on 1.6.x (compatible with Kotlin 1.9.x)
+      # Keep kotlinx-serialization on 1.7.x while the library's Kotlin API/language
+      # consumer floor is pinned to 2.0.
       - dependency-name: "org.jetbrains.kotlinx:kotlinx-serialization-json"
-        versions: [">=1.7.0"]
+        versions: [">=1.8.0"]
       # Keep Mockito on 4.x (requires JVM 1.8)
       - dependency-name: "org.mockito*:*"
         versions: [">=5.0.0"]

platforms/android/lib/build.gradle

@@ -16,7 +16,7 @@ def versionName = "4.0.0-alpha.1"
 ext {
     app_compat_version = '1.7.1'
     kotlin_stdlib_version = '2.0.21'
-    kotlin_serialization_version = '1.6.3'
+    kotlin_serialization_version = '1.7.3'
 
     androidx_test_version = '1.7.0'
     androidx_junit_ext_version = '1.3.0'