Update protoc to 3.20.1

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.

gen.go

@@ -14,7 +14,7 @@
 
 package trillian
 
-//go:generate protoc -I=. -I=$GOPATH/src -I=$GOPATH/src/github.com/googleapis/googleapis --go_out=paths=source_relative:. --go-grpc_out=paths=source_relative:. --go-grpc_opt=require_unimplemented_servers=false trillian_log_api.proto trillian_admin_api.proto trillian.proto --doc_out=markdown,api.md:./docs/
+//go:generate protoc -I=. -I=$GOPATH/src/github.com/googleapis/googleapis --go_out=paths=source_relative:. --go-grpc_out=paths=source_relative:. --go-grpc_opt=require_unimplemented_servers=false trillian_log_api.proto trillian_admin_api.proto trillian.proto --doc_out=markdown,api.md:./docs/
 //go:generate protoc -I=. --go_out=paths=source_relative:. crypto/keyspb/keyspb.proto
 
 //go:generate mockgen -package tmock -destination testonly/tmock/mock_log_server.go  github.com/google/trillian TrillianLogServer

go.mod

@@ -14,7 +14,6 @@ require (
 	github.com/go-sql-driver/mysql v1.6.0
 	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/btree v1.0.1
 	github.com/google/certificate-transparency-go v1.1.2-0.20210512142713-bed466244fa6
 	github.com/google/go-cmp v0.5.8

integration/cloudbuild/testbase/Dockerfile

@@ -26,7 +26,7 @@ RUN curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/i
 
 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 && \

quota/etcd/quotapb/gen.go

@@ -14,4 +14,4 @@
 
 package quotapb
 
-//go:generate protoc -I=. -I=$GOPATH/src -I=$GOPATH/src/github.com/googleapis/googleapis --go_out=paths=source_relative:. --go-grpc_out=paths=source_relative:. --go-grpc_opt=require_unimplemented_servers=false quotapb.proto
+//go:generate protoc -I=. -I=$GOPATH/src/github.com/googleapis/googleapis --go_out=paths=source_relative:. --go-grpc_out=paths=source_relative:. --go-grpc_opt=require_unimplemented_servers=false quotapb.proto