go.mod cleanup

[nohajc]

Out-of-sync go.mod with a reference to unused old version of protoc-gen-go-grpc causes a false positive finding in OWASP dependency-check.

Running go mod tidy fixes the issue. Also fixes #327.

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[CarstenLeue]

Hi @nohajc looks like the PR is not merged because of https://github.com/google/martian/pull/344#issuecomment-1195738697

Would you be able to comply to these requirements?

[nohajc]

Hello, this PR seems to be outdated as there are merge conflicts. The only thing I did was to run go mod tidy and submitted the result.

If this library is maintained again, I think it will be easier for you to close this PR and repeat the same steps on master (in case it's still needed).