ci: add postgres versions 11 to 15 to the build matrix

Add PostgreSQL versions 11 to 15 to the build matrix. Previously, only
PostgreSQL 12 was tested against.

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     services:
       postgres:
-        image: postgres:12
+        image: postgres:${{ matrix.postgres }}
         env:
           POSTGRES_DB: sqlalchemy_searchable_test
           POSTGRES_USER: postgres
@@ -27,20 +27,25 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - name: "Python 3.11"
+          - name: "Python 3.11, PostgreSQL 15"
             python: "3.11"
+            postgres: 15
 
-          - name: "Python 3.10"
+          - name: "Python 3.10, PostgreSQL 14"
             python: "3.10"
+            postgres: 14
 
-          - name: "Python 3.9"
+          - name: "Python 3.9, PostgreSQL 13"
             python: "3.9"
+            postgres: 13
 
-          - name: "Python 3.8"
+          - name: "Python 3.8, PostgreSQL 12"
             python: "3.8"
+            postgres: 12
 
-          - name: "PyPy"
+          - name: "PyPy, PostgreSQL 11"
             python: "pypy3.9"
+            postgres: 11
 
     steps:
       - uses: actions/checkout@v3