Kotlin: updated Kt version up-to 1.4.21

In addition, some errors during release build were solved by upgrading Gradle plugin up-to 6.7.1

app/build.gradle

@@ -117,7 +117,7 @@ dependencies {
     api 'androidx.core:core-ktx:1.3.2'
     api 'androidx.legacy:legacy-support-v4:1.0.0'
     api 'androidx.constraintlayout:constraintlayout:2.0.4'
-    testImplementation 'junit:junit:4.13'
+    testImplementation 'junit:junit:4.13.1'
     androidTestImplementation 'androidx.test.ext:junit:1.1.2'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'
 
@@ -191,7 +191,7 @@ dependencies {
     implementation 'com.android.billingclient:billing:3.0.2'
     implementation 'com.android.billingclient:billing-ktx:3.0.2'
     // https://github.com/cbeust/klaxon
-    implementation 'com.beust:klaxon:5.2'
+    implementation 'com.beust:klaxon:5.4'
     // https://github.com/SufficientlySecure/html-textview
     implementation 'org.sufficientlysecure:html-textview:4.0'
     // https://github.com/square/okhttp/tree/okhttp_3.12.x

build.gradle

@@ -1,7 +1,7 @@
 // Top-level build file where you can add configuration options common to all sub-projects/modules.
 
 buildscript {
-    ext.kotlin_version = '1.4.20'
+    ext.kotlin_version = '1.4.21'
     ext.latestAboutLibsRelease = '8.6.3'
     ext.latestFastAdapterRelease = '5.3.2'
     repositories {

gradle.properties

@@ -12,7 +12,6 @@
 # org.gradle.parallel=true
 #Thu Jun 04 12:03:50 CEST 2020
 android.enableJetifier=true
-android.enableR8.fullMode=true
 android.useAndroidX=true
 kotlin.code.style=official
 org.gradle.caching=true