build: upgrade Kotlin and turn on IR compiler

Kotlin has been building a new JVM backend for a while now
and it is finally in Beta, scheduled to hit stable in Kotlin 1.4.50.
This is the time to enable it and report any bugs we hit, before it
becomes the default.

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

ui/build.gradle

@@ -84,5 +84,6 @@ tasks.withType(JavaCompile) {
 tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
     kotlinOptions {
         jvmTarget = JavaVersion.VERSION_1_8
+        useIR = true
     }
 }