[1.7.x] Fixed #24037 -- Prevented data loss possibility when changing Meta.managed.

The migrations autodetector now issues AlterModelOptions operations for
Meta.managed changes instead of DeleteModel + CreateModel.

Thanks iambibhas for the report and Simon and Markus for review.

Backport of 061caa5b386681dc7bdef16918873043224a299c from master
[1.7.x] Fixed #24035 -- Clarified docs on CACHE_MIDDLEWARE_KEY_PREFIX vs KEY_PREFIX

Backport of 446b50b90e9e60760618b236d8b0ea75a3b19d5a from master