test(spanner): fix MutableCredentialsExample sample (#13006)

Author: sakthivelmanii

java-spanner/samples/install-without-bom/pom.xml

@@ -33,7 +33,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-spanner</artifactId>
-      <version>6.110.0</version>
+      <version>6.116.1</version>
     </dependency>
     <!-- [END spanner_install_without_bom] -->
 

java-spanner/samples/snippets/pom.xml

@@ -34,7 +34,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>libraries-bom</artifactId>
-        <version>26.76.0</version>
+        <version>26.80.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>

java-spanner/samples/snippets/src/main/java/com/example/spanner/MutableCredentialsExample.java

@@ -33,6 +33,7 @@
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
 
 public class MutableCredentialsExample {