Deprecation: add warning for required configuration

[agjohnson]

The first step that we talked about in #10342 was starting to warning projects of the upcoming changes. We definitely want to add this to the build detail page, we could also emit this as a sticky site notification on all pages. This is a more immediate change to start this process so whatever is easiest for now.

Next up, and slightly related to this, we will need to do the marketing level sort of steps. I am separating these out as these steps will come a little later than the immediate steps here.

• Deprecation: set dates for requiring configuration file and notify users #10351