Since PostgreSQL 15, DB users need to be DB owners to be able to create tables.
We can't use the new `ensureDBOwnerhip` NixOS option [1] to set this up,
because it requires the PostgreSQL user name and the database name to be
identical, which is not the case for btcpayserver.
Instead, we manually issue a PostgreSQL admin statement similar to the one
used by `ensureDBOwnerhip`.
This method of setting up the user is also compatible with older
PostgreSQL versions that come with older NixOS `system.stateVersion`s.
[1] https://github.com/NixOS/nixpkgs/pull/266270
Erik Arvstedt
770a4354b4