Downgrade Kotlin in hello-vulkan to match others.

The Kotlin use here is trivial (it might actually be better to make all
the NativeActivity-esque samples Java, since it's fewer dependencies and
less stuff in the APK, which I expect games typically want). I'll
upgrade the whole repo to something actually modern once I can do it as
a batch as opposed to repo by repo. I need all the kotlin versions
matching before I can do that though, and everything else is on 1.7.21.

hello-vulkan/build.gradle

@@ -19,5 +19,5 @@
 
 plugins {
     id 'com.android.application' version '8.4.0' apply false
-    id 'org.jetbrains.kotlin.android' version '1.8.20' apply false
+    id 'org.jetbrains.kotlin.android' version '1.7.21' apply false
 }