Bump kotlin.version from 1.7.20 to 1.7.21 in /value

Bumps `kotlin.version` from 1.7.20 to 1.7.21.
Updates `kotlin-stdlib` from 1.7.20 to 1.7.21
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Updates `kotlin-maven-plugin` from 1.7.20 to 1.7.21

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Fixes #1390

COPYBARA_INTEGRATE_REVIEW=#1390 from google:dependabot/maven/value/kotlin.version-1.7.21 7715c32
PiperOrigin-RevId: 487298186

value/src/it/functional/pom.xml

@@ -32,7 +32,7 @@
   <version>HEAD-SNAPSHOT</version>
   <name>Auto-Value Functional Integration Test</name>
   <properties>
-    <kotlin.version>1.7.20</kotlin.version>
+    <kotlin.version>1.7.21</kotlin.version>
   </properties>
   <dependencies>
     <dependency>