build: pin fmt-maven-plugin version to 2.9

fmt-maven-plugin v2.10 is compiled for java 11, v2.9 is compiled for java 8

port of fix from googleapis/java-shared-config#90

pom.xml

@@ -233,6 +233,17 @@
         </plugin>
       </plugins>
     </pluginManagement>
+    <plugins>
+      <plugin>
+        <groupId>com.coveo</groupId>
+        <artifactId>fmt-maven-plugin</artifactId>
+        <version>2.9</version>
+        <configuration>
+          <style>google</style>
+          <verbose>true</verbose>
+        </configuration>
+      </plugin>
+    </plugins>
   </build>
 
   <modules>