docs: announce removal of falcon.sys in 5.0 (#2631)

Per #2630 and review feedback on PR #2631, the deprecation warning
mechanism is omitted in favor of just announcing planned removal in
the changelog. The eager re-export of `sys` from `falcon.util` is
left in place; the actual removal will happen in 5.0.

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

Author: mvanhorn

docs/_newsfragments/2630.misc.rst

@@ -0,0 +1,4 @@
+``falcon.sys`` (an internal re-export of the standard library's
+:mod:`sys` module that was added by oversight long ago) is
+scheduled for removal in Falcon 5.0. Import the standard library's
+:mod:`sys` module directly instead.