Correct Postgres version in dev and CI

LMS's staging and production environments use Postgres 15.5 not 15.6.

.cookiecutter/cookiecutter.json

@@ -33,7 +33,7 @@
     "postgres": "yes",
     "docker": "yes",
     "__postgres_port": "5433",
-    "__postgres_version": "15.6-alpine",
+    "__postgres_version": "15.5-alpine",
     "__docker_namespace": "hypothesis",
     "__docker_network": "lms_default",
     "__github_url": "https://github.com/hypothesis/lms",

.cookiecutter/includes/.github/workflows/ci.yml

@@ -3,7 +3,7 @@ BDD_Tests:
   runs-on: ubuntu-latest
   services:
     postgres:
-      image: postgres:15.6-alpine
+      image: postgres:15.5-alpine
       ports:
       - 5433:5432
       env:

.github/workflows/ci.yml

@@ -81,7 +81,7 @@ jobs:
     runs-on: ubuntu-latest-32-cores
     services:
       postgres:
-        image: postgres:15.6-alpine
+        image: postgres:15.5-alpine
         ports:
         - 5433:5432
         env:
@@ -143,7 +143,7 @@ jobs:
     runs-on: ubuntu-latest
     services:
       postgres:
-        image: postgres:15.6-alpine
+        image: postgres:15.5-alpine
         ports:
         - 5433:5432
         env:
@@ -179,7 +179,7 @@ jobs:
     runs-on: ubuntu-latest
     services:
       postgres:
-        image: postgres:15.6-alpine
+        image: postgres:15.5-alpine
         ports:
         - 5433:5432
         env:

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   postgres:
-    image: postgres:15.6-alpine
+    image: postgres:15.5-alpine
     ports:
       - "127.0.0.1:5433:5432"
     healthcheck: