android: bump min android version

┌─ Flutter Fix ────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ The plugin google_mobile_ads requires a higher Android SDK version.                                                  │
│ Fix this issue by adding the following to the file /home/ahann/Documents/GitHub/ricochlime/android/app/build.gradle: │
│ android {                                                                                                            │
│   defaultConfig {                                                                                                    │
│     minSdkVersion 21                                                                                                 │
│   }                                                                                                                  │
│ }                                                                                                                    │
│                                                                                                                      │
│                                                                                                                      │
│ Following this change, your app will not be available to users running Android SDKs below 21.                        │
│ Consider searching for a version of this plugin that supports these lower versions of the Android SDK instead.       │
│ For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-gradle-build-configuration      │
└──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘

android/app/build.gradle

@@ -53,7 +53,7 @@ android {
         applicationId "com.adilhanney.ricochlime"
         // You can update the following values to match your application needs.
         // For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-gradle-build-configuration.
-        minSdkVersion 19
+        minSdkVersion 21
         targetSdkVersion flutter.targetSdkVersion
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName