Previous discussion: #38412, #38325, 37423e4f, 24f9c03d

- Rack::Runtime is replaced by FakeRuntime, which is a dummy middleware
  that just passes requests on and cannot be used in middleware operations
- Using Rack::Runtime in middleware operations (relative inserts, moves,
  etc.) throws a deprecation warning and uses FakeRuntime instead
- if an application adds Rack::Runtime explicitly (use, unshift, etc.),
  then the deprecation warning does not happen and FakeRuntime is
  ignored
- docs are updated to no longer reference Rack::Runtime