The legacy SDK is a very heavy dependency that this backend was only using
tiny parts of.

We'll now instead use our new "backendbase" package, which aims to provide
a smaller set of helper functions that cover the main use-cases that the
existing backends were relying on the SDK to meet, but with considerably
less code and fewer layers of abstraction.

There were two failing ACC tests with this update. One from from a newer
version of pg, which causes a different auth error message. The other is
is directly related to this PR, which was checking for a specific schema
error, which is now `invalid value for "skip_schema_creation"` when a
non-bool argument is present.