ci: update test matrix (drop postgis 3.0/3.1, add 3.5/3.6)

Author: olt

.github/workflows/ci.yml

@@ -16,11 +16,11 @@ jobs:
         go:
           - '1.22'
         postgis:
-          - '12-3.0'
-          - '13-3.1'
           - '14-3.2'
           - '15-3.3'
           - '16-3.4'
+          - '17-3.5'
+          - '18-3.6'
 
     services:
       postgres: