chore: bump fmt-maven-plugin down to 2.9.1

We support java8 and greater.
fmt-maven-plugin 2.9.x supports java8. 
fmt-maven-plugin >= 2.10 needs java11 and up.

Author: texastony

pom.xml

@@ -203,7 +203,7 @@
             <plugin>
                 <groupId>com.coveo</groupId>
                 <artifactId>fmt-maven-plugin</artifactId>
-                <version>2.11</version>
+                <version>2.9.1</version>
                 <!-- Configure no goals. -->
                 <!--Until our CI builds no longer depend on Java 8, skip this during build.-->
                 <!--We instead manually check this in Java 11 environments to block builds, -->