flutter build Runtime JAR files in the classpath should have the same version. These files were found in the classpath

In app/build.gradle change jdk7 to Java jdk8

before

dependencies {
 implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
}

after

dependencies {
 implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
}