deps: update dependency org.jetbrains.kotlin:kotlin-stdlib-jdk8 to v2.1.21 (GoogleCloudPlatform#3295)

Author: renovate-bot

pom.xml

@@ -153,7 +153,7 @@
     <dependency>
       <groupId>org.jetbrains.kotlin</groupId>
       <artifactId>kotlin-stdlib-jdk8</artifactId>
-      <version>2.1.20</version>
+      <version>2.1.21</version>
       <scope>runtime</scope>
     </dependency>
     <dependency>