Use correct Postgres and Opensearch images in CircleCI

[cgsunkel]

Description of change

When I was doing Dependabot yesterday I noticed that we were still using a Postgres 12 image in CircleCI. I've changed this to use Postgres 16 to match the actual codeabse, as well as changing the Opensearch image.

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.47%. Comparing base (0833f9d) to head (ca4f3c7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5785   +/-   ##
=======================================
  Coverage   96.46%   96.47%           
=======================================
  Files        1034     1034           
  Lines       24575    24575           
  Branches     1633     1633           
=======================================
+ Hits        23707    23708    +1     
+ Misses        711      710    -1     
  Partials      157      157           

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