Update tested PostgreSQL versions to Supported Versions

This change drops testing on PostgreSQL 9.5 and adds it on PostgreSQL 13.
This matches PostgreSQL's official Supported Versions.
You can see supported versions listed on any PostgreSQL docs page.

Author: iamed2

.github/workflows/CI.yml

@@ -45,6 +45,10 @@ jobs:
             arch: x64
             postgresql-version: latest
           # Add older supported PostgreSQL Versions
+          - os: ubuntu-latest
+            version: 1
+            arch: x64
+            postgresql-version: '13'
           - os: ubuntu-latest
             version: 1
             arch: x64
@@ -61,10 +65,6 @@ jobs:
             version: 1
             arch: x64
             postgresql-version: '9.6'
-          - os: ubuntu-latest
-            version: 1
-            arch: x64
-            postgresql-version: '9.5'
     steps:
       - uses: actions/checkout@v2
       - uses: julia-actions/setup-julia@v1