Fix Kotlin build error by updating language version

The Kotlin compiler version 1.3 is no longer supported. This was causing
build failures.

This change updates the Kotlin language version to 1.8 by adding a
`languages.yaml` configuration file for `problemtools`. This file
overrides the default Kotlin configuration and sets the language
version to 1.8.

This fixes the build error "ERROR Compile error for AC submission
Stefan.kt (Kotlin) (error: language version 1.3 is no longer
supported; please, use version 1.8 or greater.)".

Author: google-labs-jules[bot]

problems/config/languages.yaml

@@ -0,0 +1,6 @@
+kotlin:
+    name: 'Kotlin'
+    priority: 250
+    files: '*.kt'
+    compile: '/usr/bin/kotlinc -language-version 1.8 -d {path}/ -- {files}'
+    run: '/usr/bin/kotlin -Dfile.encoding=UTF-8 -J-XX:+UseSerialGC -J-Xss64m -J-Xms{memlim}m -J-Xmx{memlim}m -cp {path}/ {Mainclass}Kt'