gradle: switch the minimum SDK version to minSdk

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

We already use it to set the value to 30 for vlcBundle.

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#setMinSdk(kotlin.Int)