pw_assert: Require pw_assert CHECK backends to use the new header
pw_assert: Require pw_assert CHECK backends to use the new header

Both backend header names were supported temporarily to avoid breaking
downstream projects.

Change-Id: Ic271bebe536b5a4689175b8f3e226fc97c9ec0be
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/96760
Commit-Queue: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Reviewed-by: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed