Update Kotlin version

Signed-off-by: Ryan Nett <rnett@calpoly.edu>

Author: rnett

tensorflow-core-kotlin/pom.xml

@@ -44,7 +44,7 @@
   </dependencies>
 
   <properties>
-    <kotlin.version>1.4.20</kotlin.version>
+    <kotlin.version>1.4.21</kotlin.version>
     <kotlin.jvmTarget>1.8</kotlin.jvmTarget>
 
     <native.classifier>${javacpp.platform}${javacpp.platform.extension}</native.classifier>