Align `README` instructions between sensu/sensu-go and sensu/web

[asachs01]

Context

I believe there needs to be some alignment between the sensu/sensu-go and sensu/web repositories. The existing language seems to indicate that in order to use Sensu Go as an open-source product, that you need to build the web project. However, the comment from @portertech here: https://github.com/sensu/web/issues/370#issuecomment-800494918 indicates that we're no longer maintaining that project. So this is leading to confusion for users across the board.

The ideal situation would be that we provide very clear instructions or guidelines for our users when it comes to building and operating the OSS version of the project.

Further context to be found in the internal ref here

[echlebek]

We should update the documentation to reflect that it's possible to use sensu-go without a web ui.

[korjek]

@echlebek so is sensu/web project dead by fact or not?