Switch EPPlus to .NET Standard 2.0 This change required backporting changes from a newer EPPlus version: https://github.com/JanKallman/EPPlus/commit/ 2a0283ce04fcf9e598711c97efeff9318d7a1156 Specifically, ImageConverter (which is not .NET Standard compatible) has been replaced with an EPPlus-specific ImageCompat utility. Change-Id: Ia0a5bda9e93d97f1fad417a3a3889b0f8b69ec2b