Upgrade pitest-maven-plugin 1.11.5 -> 1.11.6 (#544)

See:
- https://github.com/hcoles/pitest/releases/tag/1.11.6
- hcoles/pitest@1.11.5...1.11.6

pom.xml

@@ -1270,7 +1270,7 @@
                 <plugin>
                     <groupId>org.pitest</groupId>
                     <artifactId>pitest-maven</artifactId>
-                    <version>1.11.5</version>
+                    <version>1.11.6</version>
                     <configuration>
                         <excludedClasses>
                             <!-- AutoValue generated classes. -->