Update protobuf-java to 3.25.6 (#1748)

* Update protobuf-java to 3.25.6

* regenerate protobuf classes with protoc 25.6

---------

Co-authored-by: scala-steward-asf[bot] <147768647+scala-steward-asf[bot]@users.noreply.github.com>
Co-authored-by: Arnout Engelen <[email protected]>

CONTRIBUTING.md

@@ -269,12 +269,14 @@ All wire protocol changes that may concern rolling upgrades should be documented
 
 Pekko includes a shaded version of protobuf `3` that is used for internal communication. To generate files,
 run `protobufGenerate`. The generated files are put in each project's `src/main/java` and need to be committed.
-The generated files are automatically transformed to use the shaded version of protobuf.
+The generated files are automatically transformed to use the shaded version of protobuf and add copyright headers.
 
-Generation depends on protoc `3.11.4` being on the path. See [protobuf project](https://github.com/protocolbuffers/protobuf#protocol-compiler-installation) for installation instructions, and
+Generation depends on the protoc version specified in `project/Dependencies.scala` being on the path. See [protobuf project](https://github.com/protocolbuffers/protobuf#protocol-compiler-installation) for installation instructions, and
 [Protobuf.scala](https://github.com/apache/pekko/blob/main/project/Protobuf.scala) for details of how to override
 the settings for generation.
 
+After generation, run `sbt javafmt Test/javafmt`.
+
 ### Pull request requirements
 
 For a pull request to be considered at all, it has to meet these requirements:

build.sbt

@@ -240,6 +240,7 @@ lazy val docs = pekkoModule("docs")
   .settings(Dependencies.docs)
   .settings(PekkoDisciplinePlugin.docs)
   .settings(Paradox.settings)
+  .settings(Protobuf.settings)
   .settings(javacOptions += "-parameters") // for Jackson
   .enablePlugins(
     ParadoxPlugin,