Radically simplify envs/

[kdmccormick]

Dupe of #34446

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random thoughts:

• envs/common should provide prod-ready defaults for as many settings as possible, which distributions like Tutor should feel comfortable using as a starting point
• envs/production is 2U-specific now since Tutor doesn't use it
• envs/devstack will become 2U-specific once the community drops devstack
• envs/bock_choy can be killed
• OEP-45's recommendation that "yaml should be used for all overrides" is unrealistic and seems to be an over-engineered way of eschewing Django's own configuration system.