Support .NET 5 for EPPlus SDK and package as a .nupkg. This change: - Adds an SDK-style project for EPPlus, which is multi-targeted - Drops support for the RIPEMD-160 hash, which is not supported in .NET 5. This is only used for (older) encrypted Excel files, which we don't support anyway. - Adds Nuget packaging support for EPPlus To generate the .nupkg, run: dotnet pack EPPlus/EPPlusSDK.csproj -c Release Change-Id: I7db0f3be1d7d28a891a61641a214a7d522ccc590