[lint] Add B028 to flake8 ignore list (ray-project#48465)

I've recently started seeing lint errors like this locally:
```
python/ray/serve/handle.py:788:13: B028 No explicit stacklevel argument found. The warn method from the warnings module uses a stacklevel of 1 by default. This
 will only show a stack trace for the line on which the warn method is called. It is therefore recommended to use a stacklevel of 2 or greater to provide more
information to the user.
```

We use `warnings` for deprecation messages and don't want to include
stack traces in them, so ignoring this lint error.

Signed-off-by: Edward Oakes <ed.nmi.oakes@gmail.com>

.flake8

@@ -14,6 +14,7 @@ exclude =
 max-line-length = 88
 inline-quotes = "
 ignore =
+  B028
   C408
   C417
   E121