add junit.version property to ensure version updates by renovate-bot

Author: nidhiii-27

java-storage/gapic-google-cloud-storage-v2/pom.xml

@@ -98,7 +98,7 @@
                             <dependency>
                                 <groupId>org.junit.vintage</groupId>
                                 <artifactId>junit-vintage-engine</artifactId>
-                                <version>5.14.3</version>
+                                <version>${junit.version}</version>
                             </dependency>
                         </dependencies>
                     </plugin>

java-storage/google-cloud-storage-control/pom.xml

@@ -115,7 +115,7 @@
                             <dependency>
                                 <groupId>org.junit.vintage</groupId>
                                 <artifactId>junit-vintage-engine</artifactId>
-                                <version>5.14.3</version>
+                                <version>${junit.version}</version>
                             </dependency>
                         </dependencies>
                     </plugin>

java-storage/pom.xml

@@ -56,14 +56,15 @@
     <github.global.server>github</github.global.server>
     <site.installationModule>google-cloud-storage-parent</site.installationModule>
     <google.cloud.shared-dependencies.version>3.31.0</google.cloud.shared-dependencies.version>
+    <junit.version>5.14.3</junit.version>
   </properties>
 
   <dependencyManagement>
     <dependencies>
       <dependency>
         <groupId>org.junit</groupId>
         <artifactId>junit-bom</artifactId>
-        <version>5.14.3</version>
+        <version>${junit.version}</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>