generated-files: update protoc to v3.11.4 to match google/protobuf

see https://github.com/golang/protobuf/blob/v1.3.5/.travis.yml#L15

Note that gogo/protobuf v1.3.2 updated protoc to v3.14.0; https://github.com/gogo/protobuf/releases/tag/v1.3.2
however, regenerating protobufs with this switched an import to use
google.golang.org/protobuf/types/known/timestamppb instead of
github.com/golang/protobuf/ptypes/timestamp, so using the older
version for now

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/dockerfiles/generated-files.Dockerfile

@@ -2,7 +2,8 @@
 
 # protoc is dynamically linked to glibc to can't use golang:1.10-alpine
 FROM golang:1.13-buster AS gobuild-base
-ARG PROTOC_VERSION=3.1.0
+# https://github.com/golang/protobuf/blob/v1.3.5/.travis.yml#L15
+ARG PROTOC_VERSION=3.11.4
 ARG GOGO_VERSION=v1.3.2
 RUN apt-get update && apt-get --no-install-recommends install -y \
 	unzip \