chore: upgrade protobuf version to v3.25.8 (GoogleCloudPlatform#3831)

Author: zhumin8

spring-cloud-generator/pom.xml

@@ -18,7 +18,7 @@
         <gapic-generator-java-bom.version>2.59.0</gapic-generator-java-bom.version>
         <!-- [gapic-test]: protobuf.version for compiling protos used in unit testing
              this should be consistent with version in gapic-generator-java-bom -->
-        <protobuf.version>3.25.3</protobuf.version>
+        <protobuf.version>3.25.8</protobuf.version>
     </properties>
 
     <dependencyManagement>