chore: min sdk testapp

Author: MichaelVerdon

tests/android/app/build.gradle

@@ -41,7 +41,7 @@ android {
     defaultConfig {
         applicationId = "io.flutter.plugins.firebase.tests"
         // auth requires minSdk 23
-        minSdk = 23
+        minSdkVersion flutter.minSdkVersion
         targetSdk = 35
         versionCode = flutterVersionCode.toInteger()
         versionName = flutterVersionName