Update to protobuf v1.25 and grpc 1.35; bump CI to go1.12

From:
github.com/golang/protobuf v1.3.2
google.golang.org/grpc v1.17.0

To:
github.com/golang/protobuf v1.4.3
google.golang.org/grpc v1.35.0
google.golang.org/protobuf v1.25.0

About the two protobuf packages:
https://developers.google.com/protocol-buffers/docs/reference/go/faq
> Version v1.4.0 and higher of github.com/golang/protobuf wrap the new
implementation and permit programs to adopt the new API incrementally. For
example, the well-known types defined in github.com/golang/protobuf/ptypes are
simply aliases of those defined in the newer module. Thus,
google.golang.org/protobuf/types/known/emptypb and
github.com/golang/protobuf/ptypes/empty may be used interchangeably.

Notable Code Changes in zrepl:
- generate protobufs now contain a mutex so we can't copy them by value
  anymore
- grpc.WithDialer is deprecated => use grpc.WithContextDialer instead

Go1.12 is now actually required by some of the dependencies.

build/go.mod

@@ -4,7 +4,6 @@ go 1.12
 
 require (
 	github.com/alvaroloes/enumer v1.1.1
-	github.com/golang/protobuf v1.4.2
 	github.com/golangci/golangci-lint v1.35.2
 	github.com/golangci/misspell v0.3.4 // indirect
 	github.com/golangci/revgrep v0.0.0-20180812185044-276a5c0a1039 // indirect
@@ -12,4 +11,6 @@ require (
 	github.com/spf13/jwalterweatherman v1.1.0 // indirect
 	github.com/wadey/gocovmerge v0.0.0-20160331181800-b5bfa59ec0ad
 	golang.org/x/tools v0.0.0-20210105210202-9ed45478a130
+	google.golang.org/grpc/cmd/protoc-gen-go-grpc v1.1.0 // indirect
+	google.golang.org/protobuf v1.25.0
 )