Test on CircleCI using PostgreSQL 13

[david-a-wheeler]

Modify the CircleCI configuration so that we test using PostgreSQL 13. We're about to switch from 12->13, and we want to ensure that (1) it works and (2) we make the test environment maximally similar to production.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.04%. Comparing base (457239f) to head (1951490).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2128 +/- ## ======================================= Coverage 98.04% 98.04% ======================================= Files 53 53 Lines 2099 2099 ======================================= Hits 2058 2058 Misses 41 41 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.