Upgrade Kotlin patch version to 1.7.21

Author: seratch

pom.xml

@@ -52,7 +52,7 @@
         <jakarta.servlet-api.version>5.0.0</jakarta.servlet-api.version>
         <okhttp.version>4.10.0</okhttp.version>
         <gson.version>2.10</gson.version>
-        <kotlin.version>1.7.20</kotlin.version>
+        <kotlin.version>1.7.21</kotlin.version>
         <!-- Note that we should not upgrade sfl4j-api to v2 to avoid confusions on the users side.
             see also: https://github.com/slackapi/java-slack-sdk/issues/1034
         -->