Update version to Pre-Release v2.1
Update version to Pre-Release v2.1

LordZoltan/docfx,DuncanmaMSFT/docfx,pascalberger/docfx,superyyrrzz/docfx,928PJY/docfx,pascalberger/docfx,pascalberger/docfx,hellosnow/docfx,hellosnow/docfx,928PJY/docfx,dotnet/docfx,hellosnow/docfx,928PJY/docfx,dotnet/docfx,dotnet/docfx,LordZoltan/docfx,LordZoltan/docfx,DuncanmaMSFT/docfx,superyyrrzz/docfx,LordZoltan/docfx,superyyrrzz/docfx