In preperation for deprecating non-project mode at some point. This is
phase 1 of encouraging users off of legacy mode.
Any time a user opens a backend in legacy mode (either old states, or
new states with SELF_MANAGED_STATE_LEGACY_LAYOUT set) we'll warn that
non-project mode is due to deprecate this year.
They can disable this warning by setting
PULUMI_DIY_BACKEND_ACKNOWLEDGE_LEGACY_WARNING to true.
The next phase (in a few months) will turn this into an error which
they'll be able to opt out of by setting
PULUMI_DIY_BACKEND_ACKNOWLEDGE_LEGACY_ERROR to true.
After that it will be a hard error and all non-project related code will
be removed.