[backend] ci: postgres versions are out of date

[alxndrsn]

PostgreSQL version in production was updated in https://github.com/getodk/central/pull/1546/changes#diff-6cd399313d6a6060d55ba88501a5c330672b41614330778b91c26b67b49c1a64L1-R1

Versions in CI are still 14.10:

• https://github.com/getodk/central-backend/blob/97981e9694f392e5b3df90a2d28c6f7d3aefef17/.github/workflows/standard-suite.yml#L15
• https://github.com/getodk/central-backend/blob/97981e9694f392e5b3df90a2d28c6f7d3aefef17/.circleci/config.yml#L6