Update dependencies: efcore to 3.0.0-preview.18604.3, extensions to 3.0.0-preview.18605.1

[natemcmaster]

cc @pakrym @sebastienros

[pakrym]

🤞

[natemcmaster]

Getting a weird compiler error.

https://dnceng.visualstudio.com/public/_build/results?buildId=53319&view=logs&jobId=f31c9f97-4411-58e7-49ac-fc73f645e6b6&taskId=2bcaa12b-2f4b-5b1f-c519-10308f653190&lineStart=4707&lineEnd=4709&colStart=1&colEnd=32

2018-12-06T01:24:47.7513578Z   D:\a\1\s\src\AuthSamples\samples\StaticFilesAuth\Startup.cs(128,36): error CS0246: The type or namespace name 'List<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\1\s\src\AuthSamples\samples\StaticFilesAuth\StaticFilesAuth.csproj]
2018-12-06T01:24:47.7609318Z   D:\a\1\s\src\AuthSamples\samples\StaticFilesAuth\Startup.cs(137,25): error CS1061: 'HttpContext' does not contain a definition for 'SetEndpoint' and no accessible extension method 'SetEndpoint' accepting a first argument of type 'HttpContext' could be found (are you missing a using directive or an assembly reference?) [D:\a\1\s\src\AuthSamples\samples\StaticFilesAuth\StaticFilesAuth.csproj]
2018-12-06T01:24:47.8081088Z