Upgrade Checkstyle 10.12.5 -> 10.12.6 (#911)

See:
- https://checkstyle.sourceforge.io/releasenotes.html
- https://github.com/checkstyle/checkstyle/releases/tag/checkstyle-10.12.6
- checkstyle/checkstyle@checkstyle-10.12.5...checkstyle-10.12.6

pom.xml

@@ -859,7 +859,7 @@
                         <dependency>
                             <groupId>com.puppycrawl.tools</groupId>
                             <artifactId>checkstyle</artifactId>
-                            <version>10.12.5</version>
+                            <version>10.12.6</version>
                         </dependency>
                         <dependency>
                             <groupId>io.spring.nohttp</groupId>