make logback version consistent

Author: JoeWang1127

java-showcase/gapic-showcase/pom.xml

@@ -18,6 +18,7 @@
 
   <properties>
     <gapic-showcase.version>0.35.1</gapic-showcase.version>
+    <slf4j1-logback.version>1.2.13</slf4j1-logback.version>
   </properties>
 
   <build>
@@ -281,13 +282,13 @@
         <dependency>
           <groupId>ch.qos.logback</groupId>
           <artifactId>logback-classic</artifactId>
-          <version>1.2.13</version>
+          <version>${slf4j1-logback.version}</version>
           <scope>test</scope>
         </dependency>
         <dependency>
           <groupId>ch.qos.logback</groupId>
           <artifactId>logback-core</artifactId>
-          <version>1.3.15</version>
+          <version>${slf4j1-logback.version}</version>
           <scope>test</scope>
         </dependency>
       </dependencies>