Increase minSdkVersion to 21

Author: OlegNitz

packages/core/src/lib/TwaManifest.ts

@@ -72,7 +72,7 @@ const DEFAULT_NAVIGATION_DIVIDER_COLOR = '#00000000';
 const DEFAULT_BACKGROUND_COLOR = '#FFFFFF';
 const DEFAULT_APP_VERSION_CODE = 1;
 const DEFAULT_APP_VERSION_NAME = DEFAULT_APP_VERSION_CODE.toString();
-const DEFAULT_MIN_SDK_VERSION = 19;
+const DEFAULT_MIN_SDK_VERSION = 21;
 const DEFAULT_SIGNING_KEY_PATH = './android.keystore';
 const DEFAULT_SIGNING_KEY_ALIAS = 'android';
 const DEFAULT_ENABLE_NOTIFICATIONS = true;