Update protoc to v3.20.1 (#923)

Since v3.14.0, protoc started using the new import path for Go types:
google.golang.org/protobuf. This change updates the protobuf compiler
to the latest v3.20.1, to get rid of the deprecated import path and take
advantage of newer versions after v3.14.0.

.github/dependabot.yml

@@ -17,11 +17,7 @@ updates:
     - 1.10.0
   - dependency-name: google.golang.org/protobuf
     versions:
-    - 1.26.0
-  - dependency-name: github.com/golang/protobuf
-    versions:
-    - 1.5.1
-    - 1.5.2
+    - 1.28.0
   - dependency-name: github.com/golang/mock
     versions:
     - 1.5.0

go.mod

@@ -7,7 +7,6 @@ require (
 	github.com/fullstorydev/grpcurl v1.8.6
 	github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b
 	github.com/golang/mock v1.6.0
-	github.com/golang/protobuf v1.5.2
 	github.com/google/go-cmp v0.5.8
 	github.com/google/trillian v1.4.1-0.20220503161811-32257c5c3ca0
 	github.com/gorilla/mux v1.8.0

integration/Dockerfile

@@ -14,7 +14,7 @@ RUN cd /usr/bin && curl -L -O https://github.com/stedolan/jq/releases/download/j
 RUN curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $(go env GOPATH)/bin v1.18.0
 RUN mkdir protoc && \
     (cd protoc && \
-    PROTOC_VERSION=3.12.4 && \
+    PROTOC_VERSION=3.20.1 && \
     PROTOC_ZIP="protoc-${PROTOC_VERSION}-linux-x86_64.zip" && \
     wget "https://github.com/google/protobuf/releases/download/v${PROTOC_VERSION}/${PROTOC_ZIP}" && \
     unzip -o ${PROTOC_ZIP} -d /usr/local bin/protoc && \