Your project requires a newer version of the Kotlin Gradle plugin. Find the latest version on https://kotlinlang.org/docs/gradle.html#plugin-and-versions, then update project/android/build.gradle: ext.kotlin_version = ''
But You have already changed the "ext.kotlin_version = ''"
and still facing the issue, Here you can follow the steps to solve this problem,
Note :-Editing the Kotlin version in only gradle.build not helps in most cases and you have to edit also in your setting.gradle file and edit your Kotlin version according to your Gradle version

Step-1
Find the version of your gradle , in your Flutter project

Step-2
Visit this link https://kotlinlang.org/docs/gradle-configure-project.html#apply-the-plugin
and find suitable Kotlin version according to your Gradle version.

Step-3
go to project folder/android/gradle.build
and write this and if this is already written just edit it according to your gradle version

ext.kotlin_version = 'x.y.z'

in your build.gradle file

Now go to setting.gradle file and write the Kotlin version as same as you have written in build.gradle file

Thank you for reading this will for works for you