Merge pull request #975: [infra] revert protobufs to 3.x

Author: je-ik

buildSrc/src/main/groovy/cz.o2.proxima.java-conventions.gradle

@@ -27,7 +27,7 @@ def auto_service_version = "1.1.1"
 def beam_version = "2.64.0"
 def errorprone_version = "2.18.0"
 def flink_version = "1.18.0"
-def grpc_version = "1.70.0"
+def grpc_version = "1.69.0"
 def groovy_version = "4.0.24"
 def guava_version = "33.1.0-jre"
 def hadoop_version = "3.2.4"
@@ -38,7 +38,7 @@ def junit4_version = "4.13.2"
 def lombok_version = "1.18.28"
 def log4j_version = "2.19.0"
 def mockito_version = "5.3.0"
-def protobuf_version = "4.29.0"
+def protobuf_version = "3.25.5"
 def scala_version = "2.12"
 def spark_version = "3.1.2"
 def slf4j_version = "2.0.7"