Bump checkstyle from 6.19 to 8.29 in /clients

Bumps [checkstyle](https://github.com/checkstyle/checkstyle) from 6.19 to 8.29.
- [Release notes](https://github.com/checkstyle/checkstyle/releases)
- [Commits](checkstyle/checkstyle@checkstyle-6.19...checkstyle-8.29)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

clients/pom.xml

@@ -95,7 +95,7 @@
     <maven-surefire-plugin.version>2.21.0</maven-surefire-plugin.version>
     <os-maven-plugin.version>1.4.1.Final</os-maven-plugin.version>
     <protobuf-maven-plugin.version>0.6.1</protobuf-maven-plugin.version>
-    <puppycrawl.checkstyle.version>6.19</puppycrawl.checkstyle.version>
+    <puppycrawl.checkstyle.version>8.29</puppycrawl.checkstyle.version>
     <spotbugs-maven-plugin.version>3.1.8</spotbugs-maven-plugin.version>
   </properties>