Test against PostgreSQL 15

[clokep]

https://www.postgresql.org/docs/15/release-15.html

We should update our CI to test against latest Postgres, which is now 15 instead of 14.

[clokep]

I think the steps to do this are:

1. Update the tests.yml file and replace 14 with 15.

[realtyem]

Hold off for a bit, newest thing up there is the RC2 version

https://hub.docker.com/_/postgres

[realtyem]

Ready now 😄

[squahtx]

I think the steps to do this are:

1. Update the tests.yml file and replace 14 with 15.

2. Update .ci/scripts/calculate_jobs.py and replace 14 with 15.