gradle: switch the target SDK version to targetSdk

The difference between targetSdk and targetSdkVersion seems that the former is an integer [^1].

We keep the value in rootProject.ext as it's currenly used by libvlcjni.

[^1]: https://developer.android.com/reference/tools/gradle-api/8.3/null/com/android/build/api/dsl/SettingsExtension#setCompileSdk(kotlin.Int)