Update docs related to Postgres migration

[matthew-white]

Update docs related to Postgres and the Postgres migration (getodk/central#343):

• Users will need to follow different upgrade instructions for v2023.1, at least if they're using the default Postgres setup.
• Other docs related to Postgres will probably need to change. For example:
  • https://github.com/getodk/docs/blame/2bf95e63b767a127eb4af7f796b3f026d2357b64/docs/central-install-digital-ocean.rst#L270
  • https://github.com/getodk/docs/blame/2bf95e63b767a127eb4af7f796b3f026d2357b64/docs/central-install-digital-ocean.rst#L323