fix: Update minSdkVersion to 21 to Fix NDK Compatibility Issue (#1832)

Flutter 3.22 had dropped support for Android KitKat https://docs.flutter.dev/release/breaking-changes/android-kitkat-deprecation, update minSdkVersion to 21 to Fix NDK Compatibility Issue.

android/build.gradle

@@ -30,7 +30,7 @@ android {
     compileSdkVersion safeExtGet('compileSdkVersion', 31)
 
     defaultConfig {
-        minSdkVersion safeExtGet('minSdkVersion', 16)
+        minSdkVersion safeExtGet('minSdkVersion', 21)
 
         consumerProguardFiles 'consumer-rules.pro'
     }