Downgrade protobuf version to 3.25.5

This still fixes GHSA-735f-pc8j-v9w8, but avoids a breaking change to 4.x, see #4634

PiperOrigin-RevId: 689792648

Author: cushon

pom.xml

@@ -43,7 +43,7 @@
     <maven-javadoc-plugin.version>3.3.1</maven-javadoc-plugin.version>
     <maven-source-plugin.version>3.2.1</maven-source-plugin.version>
     <nexus-staging-maven-plugin.version>1.6.13</nexus-staging-maven-plugin.version>
-    <protobuf.version>4.28.2</protobuf.version>
+    <protobuf.version>3.25.5</protobuf.version>
     <grpc.version>1.43.3</grpc.version>
     <jspecify.version>1.0.0</jspecify.version>
   </properties>