default zeitwerk to on unless kill-switched
default zeitwerk to on unless kill-switched

refs FOO-2443
flag=none

TEST PLAN:
  1) specs pass

Change-Id: I7b336ef83dbe7da4db257f6b4e2da42cecb9a8fe
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/277167
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Jacob Burroughs <8ecea6e385af5cf9f53123f5ca17fb5fd6a6d4b2@instructure.com>
QA-Review: Ethan Vizitei <73eeb0b2f65d05a906adf3b21ee1f9f5a2aa3c1c@instructure.com>
Product-Review: Ethan Vizitei <73eeb0b2f65d05a906adf3b21ee1f9f5a2aa3c1c@instructure.com>

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