With the 2.8 release where the minimum and target SDK versions is set/established has changed within the Flutter project. How to change Android minSdkVersion in flutter project? There are a couple approaches provided below.

1. You can change the minSdkVersion directly in the file Project_Name/android/app/build.gradle , defaultconfig

       defaultConfig {
       // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
       applicationId "com.example.projectname"
       minSdkVersion 16 // <--- There
       targetSdkVersion 27
       versionCode 1
       versionName "1.0"
       testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"}
   

2. The more appropriate way and maintainable approach is a little more involved however I would recommend.

   For clarity I will provide what the build.gradle looked like before and after the 2.8 update.

       android {
           compileSdkVersion 30
       
       defaultConfig {
           applicationId "com.example.app"
           minSdkVersion 21
           targetSdkVersion 30
           versionCode flutterVersionCode.toInteger()
           versionName flutterVersionName
           multiDexEnabled true
       }        
   

   After updating to Flutter 2.8:

       android {
           compileSdkVersion flutter.compileSdkVersion
       
       defaultConfig {
               applicationId "com.example.app"
               minSdkVersion flutter.minSdkVersion
               targetSdkVersion flutter.targetSdkVersion
               versionCode flutterVersionCode.toInteger()
               versionName flutterVersionName
           }
   

So here are the changes so that you can maintain the version(s) within your project.

Update the android -> local.properties adding the following 3 lines. This will be where you set the min and target versions going forward.

    flutter.minSdkVersion=21
    flutter.targetSdkVersion=30
    flutter.compileSdkVersion=30

Add/modify to the android/src/build.gradle file

    android {
        compileSdkVersion localProperties.getProperty('flutter.compileSdkVersion').toInteger()
    ...
    defaultConfig {
        minSdkVersion localProperties.getProperty('flutter.minSdkVersion').toInteger()
        targetSdkVersion localProperties.getProperty('flutter.targetSdkVersion').toInteger()
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
    }

Keep in mind with this appraoch going forward you will maintain minimum and target SDK versions within the local.properties file.