ci: use postgres 15 for content api tests (#16706)

Content API Tests are currently failing because this job boots the
latest Figma-hosted Content API image against `postgres:13`. The latest
image includes https://github.com/figma/figma/pull/776922, which added
PG15-only `UNIQUE NULLS NOT DISTINCT` index syntax, so Content API
migrations fail before Payload tests run. cc @asalani-figma

Failed job example:
- Content API Tests on #16702:
https://github.com/payloadcms/payload/actions/runs/26216770563/job/77142295149?pr=16702

This updates only the ephemeral database used by the Content API service
in Payload CI. That database belongs to Figma-hosted Content API, not
Payload or the database adapter, so matching Content API's Postgres
requirement is the intended fix here.

---------

Co-authored-by: German Jablonski <GermanJablo@users.noreply.github.com>

Author: GermanJablo

.github/workflows/main.yml

@@ -502,7 +502,7 @@ jobs:
       contents: read
     services:
       postgres:
-        image: postgres:13
+        image: postgres:15
         env:
           POSTGRES_DB: figma_content_api
           POSTGRES_USER: postgres