[backend] circleci: bump postgres version to 14.20

[alxndrsn]

Blocked

• Feature Request: 14.20 image CircleCI-Public/cimg-postgres#148

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In getodk/central-backend#1719, CircleCI postgres version in https://github.com/getodk/central-backend/blob/master/.circleci/config.yml was bumped to v14.19 instead of v14.20.

This is because CircleCI has not published a test image for postgres v14.20: CircleCI-Public/cimg-postgres#148

Related: #1572