Remove Postgres 14 and add 18 to test matrix

Author: filiphr

.github/workflows/postgres.yml

@@ -19,7 +19,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        postgres: [14, 15, 16, 17]
+        postgres: [15, 16, 17, 18]
     services:
       postgres:
         image: postgres:${{ matrix.postgres }}