chore: update and fix logback versions used in test (#3990)

These are test only dependencies, dependabot
[alerts](https://github.com/googleapis/sdk-platform-java/security/dependabot?q=package%3Ach.qos.logback%3Alogback-core+manifest%3Ajava-showcase%2Fgapic-showcase%2Fpom.xml+has%3Apatch)
should not matter as code is not shipped and test is in controlled env.
However, it is good practice to update versions when possible.

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: zhumin8

java-showcase/gapic-showcase/pom.xml

@@ -18,7 +18,9 @@
 
   <properties>
     <gapic-showcase.version>0.36.2</gapic-showcase.version>
+    <!-- This is the last version supporting slf4j 1.x, do not upgrade -->
     <slf4j1-logback.version>1.2.13</slf4j1-logback.version>
+    <slf4j2-logback.version>1.5.21</slf4j2-logback.version>
   </properties>
 
   <build>
@@ -244,13 +246,13 @@
         <dependency>
           <groupId>ch.qos.logback</groupId>
           <artifactId>logback-classic</artifactId>
-          <version>1.5.16</version>
+          <version>${slf4j2-logback.version}</version>
           <scope>test</scope>
         </dependency>
         <dependency>
           <groupId>ch.qos.logback</groupId>
           <artifactId>logback-core</artifactId>
-          <version>1.5.16</version>
+          <version>${slf4j2-logback.version}</version>
           <scope>test</scope>
         </dependency>
       </dependencies>
@@ -333,13 +335,13 @@
         <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>