Upgrade gradle version of the kotlinx-benchmark plugin to 6.8

Author: Abduqodiri Qurbonzoda

plugin/gradle/wrapper/gradle-wrapper.properties

@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.3-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.8-all.zip