Update CodeCoverage to use .NETCORE2.1

tests/CodeCoverage/CodeCoverage.cmd

@@ -12,7 +12,7 @@ dotnet restore ImageSharp.sln
 rem Clean the solution to force a rebuild with /p:codecov=true
 dotnet clean ImageSharp.sln -c Release
 rem The -threshold options prevents this taking ages...
-tests\CodeCoverage\OpenCover.4.6.519\tools\OpenCover.Console.exe -target:"dotnet.exe" -targetargs:"test tests\ImageSharp.Tests\ImageSharp.Tests.csproj -c Release -f netcoreapp2.0 /p:codecov=true" -register:user -threshold:10 -oldStyle -safemode:off -output:.\ImageSharp.Coverage.xml -hideskipped:All -returntargetcode -filter:"+[SixLabors.ImageSharp*]*" 
+tests\CodeCoverage\OpenCover.4.6.519\tools\OpenCover.Console.exe -target:"dotnet.exe" -targetargs:"test tests\ImageSharp.Tests\ImageSharp.Tests.csproj -c Release -f netcoreapp2.1 /p:codecov=true" -register:user -threshold:10 -oldStyle -safemode:off -output:.\ImageSharp.Coverage.xml -hideskipped:All -returntargetcode -filter:"+[SixLabors.ImageSharp*]*" 
 
 if %errorlevel% neq 0 exit /b %errorlevel%