upgrade to pg v15

This reverts commit 40a5fbd.

.circleci/config.yml

@@ -15,7 +15,7 @@ jobs:
           DATABASE_URL: postgres://[email protected]/cfdm_cms_test
 
       # PostgreSQL
-      - image: cimg/postgres:13.8
+      - image: cimg/postgres:15.6
         environment:
           POSTGRES_USER: postgres
           POSTGRES_HOST_AUTH_METHOD: "trust"
@@ -31,7 +31,7 @@ jobs:
           # https://circleci.com/docs/2.0/postgres-config/#postgresql-circleci-configuration-example
           command: |
             sudo apt-get update -qq && sudo apt-get install -y build-essential postgresql-client
-            echo 'export PATH=/usr/lib/postgresql/13.8/bin/:$PATH' >> $BASH_ENV
+            echo 'export PATH=/usr/lib/postgresql/15.6/bin/:$PATH' >> $BASH_ENV
             echo "en_US.UTF-8 UTF-8" | sudo tee /etc/locale.gen
             sudo locale-gen en_US.UTF-8
 

README.md

@@ -59,7 +59,7 @@ run into problems please
     * Python (the latest 3.10 release, which includes `pip` and and a built-in version of `virtualenv` called `venv`).
     * The latest long term support (LTS) or stable release of Node.js (which
       includes `npm`).
-    * PostgreSQL (the latest 13 release).
+    * PostgreSQL (the latest 15 release).
          * Read a [Mac OSX tutorial](https://www.moncefbelyamani.com/how-to-install-postgresql-on-a-mac-with-homebrew-and-lunchy/).
          * Read a [Windows tutorial](https://www.postgresqltutorial.com/install-postgresql/).
          * Read a [Linux tutorial](https://www.postgresql.org/docs/13/installation.html)