With the new release os VS, it's required that all plugins migrate the new VSIX manifest format. The new format is backwards compatible with all versions newer that Visual Studio 2012, so this migration effectively drops support for older versions of the IDE.
It's also required that these new extensions are built with Visual Studio 2017, so unfortunately it was necessary to migrate the project and solution. Also removed COM references to EnvDTE and Microsoft.VisualStudio.TextManager.Interop from the csproj, as they seem to both be unnecessary and would trigger build warnings because of changes to GAC.