Bump Google.Protobuf from 3.8.0 to 3.15.0 in /src/Eventuate#3

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dependabot/nuget/src/Eventuate/Google.Protobuf-3.15.0Horusiath/Eventuate.NET:dependabot/nuget/src/Eventuate/Google.Protobuf-3.15.0Copy head branch name to clipboard