Bump checkstyle from 8.9 to 8.18 (#556)

diff --git a/pom.xml b/pom.xml
index 155805b..ffeee72 100644
--- a/pom.xml
+++ b/pom.xml
@@ -209,7 +209,7 @@
 							<dependency>
 								<groupId>com.puppycrawl.tools</groupId>
 								<artifactId>checkstyle</artifactId>
-								<version>8.9</version>
+								<version>8.18</version>
 							</dependency>
 						</dependencies>
 						<executions>