fix: Downgrade Protobuf from 4.x to 3.x

Motivation:

- grpc-java is not fully compatible with Protobuf 4; see: grpc/grpc-java#11015
- Protobuf 4 breaks our demo server due to an incompatibility between the generated code and what the grpc-java library expects.

Modifications:

- Downgrade to Protobuf 3 on all projects.
- Prevent upgrade to Protobuf 4 in dependabot.yml (note: scala-steward doesn't seem to pick up Protobuf updates anyway, probably because it's not configured as a library dependency per se).

Result:

Stick to Protobuf 3 for now.

.github/dependabot.yml

@@ -7,13 +7,19 @@ updates:
     schedule:
       interval: "weekly"
       day: "sunday"
+    ignore:
+      - dependency-name: "com.google.protobuf:protoc"
+        versions: ['>= 4'] # https://github.com/grpc/grpc-java/issues/11015
   - package-ecosystem: "maven"
     directory: "/java/maven"
     commit-message:
       prefix: "chore"
     schedule:
       interval: "weekly"
       day: "sunday"
+    ignore:
+      - dependency-name: "com.google.protobuf:protoc"
+        versions: ['>= 4'] # https://github.com/grpc/grpc-java/issues/11015
   - package-ecosystem: "gradle"
     directory: "/kotlin/gradle"
     commit-message:
@@ -25,13 +31,19 @@ updates:
     schedule:
       interval: "weekly"
       day: "sunday"
+    ignore:
+      - dependency-name: "com.google.protobuf:protoc"
+        versions: ['>= 4'] # https://github.com/grpc/grpc-java/issues/11015
   - package-ecosystem: "maven"
     directory: "/kotlin/maven"
     commit-message:
       prefix: "chore"
     schedule:
       interval: "weekly"
       day: "sunday"
+    ignore:
+      - dependency-name: "com.google.protobuf:protoc"
+        versions: ['>= 4'] # https://github.com/grpc/grpc-java/issues/11015
   - package-ecosystem: "gradle"
     directory: "/server"
     commit-message:
@@ -43,3 +55,6 @@ updates:
     schedule:
       interval: "weekly"
       day: "sunday"
+    ignore:
+      - dependency-name: "com.google.protobuf:protoc"
+        versions: ['>= 4'] # https://github.com/grpc/grpc-java/issues/11015

java/gradle/build.gradle

@@ -43,7 +43,7 @@ sourceSets {
 
 protobuf {
   protoc {
-    artifact = "com.google.protobuf:protoc:4.26.0"
+    artifact = "com.google.protobuf:protoc:3.25.3"
   }
   plugins {
     grpc {

java/maven/pom.xml

@@ -19,7 +19,7 @@
         <gatling-maven-plugin.version>4.8.2</gatling-maven-plugin.version>
         <maven-compiler-plugin.version>3.13.0</maven-compiler-plugin.version>
         <maven-jar-plugin.version>3.3.0</maven-jar-plugin.version>
-        <protobuf.version>3.24.2</protobuf.version>
+        <protobuf.version>3.25.3</protobuf.version>
         <protobuf-maven-plugin.version>0.6.1</protobuf-maven-plugin.version>
         <protoc-gen-grpc-java.version>1.57.2</protoc-gen-grpc-java.version>
         <spotless.version>2.43.0</spotless.version>

kotlin/maven/pom.xml

@@ -21,7 +21,7 @@
         <maven-compiler-plugin.version>3.13.0</maven-compiler-plugin.version>
         <maven-jar-plugin.version>3.3.0</maven-jar-plugin.version>
         <maven-resources-plugin.version>3.3.1</maven-resources-plugin.version>
-        <protobuf.version>4.26.0</protobuf.version>
+        <protobuf.version>3.25.3</protobuf.version>
         <protobuf-maven-plugin.version>0.6.1</protobuf-maven-plugin.version>
         <protoc-gen-grpc-java.version>1.57.2</protoc-gen-grpc-java.version>
         <spotless.version>2.43.0</spotless.version>

server/build.gradle

@@ -27,7 +27,7 @@ idea {
 
 protobuf {
   protoc {
-    artifact = "com.google.protobuf:protoc:4.26.0"
+    artifact = "com.google.protobuf:protoc:3.25.3"
   }
   plugins {
     grpc {