Add support for github actions for new branch PG18

While on it, remove the tests for PG12 as this branch is out of support.

Backpatch-through: 13

Author: michaelpq

.github/workflows/test.yml

@@ -2,20 +2,20 @@ name: regression test
 on:
   push:
     branches:
+      - PG18
       - PG17
       - PG16
       - PG15
       - PG14
       - PG13
-      - PG12
   pull_request:
     branches:
+      - PG18
       - PG17
       - PG16
       - PG15
       - PG14
       - PG13
-      - PG12
   schedule:
     # Runs at 00:00 UTC on every Sunday.
     - cron: "0 0 * * SUN"
@@ -37,7 +37,9 @@ jobs:
 
       - name: Set PATH and PG_VERSION
         run: |
-          if [ "${{ github.ref_name }}" == 'PG17' ]; then
+          if [ "${{ github.ref_name }}" == 'PG18' ]; then
+            echo "PG_VERSION=18" >> $GITHUB_ENV
+          elif [ "${{ github.ref_name }}" == 'PG17' ]; then
             echo "PG_VERSION=17" >> $GITHUB_ENV
           elif [ "${{ github.ref_name }}" == 'PG16' ]; then
             echo "PG_VERSION=16" >> $GITHUB_ENV
@@ -47,8 +49,6 @@ jobs:
             echo "PG_VERSION=14" >> $GITHUB_ENV
           elif [ "${{ github.ref_name }}" == 'PG13' ]; then
             echo "PG_VERSION=13" >> $GITHUB_ENV
-          elif [ "${{ github.ref_name }}" == 'PG12' ]; then
-            echo "PG_VERSION=12" >> $GITHUB_ENV
           fi
 
       - name: Build PostgreSQL