Update PostGIS Docker image tags in workflow

Author: mprins

.github/workflows/linux.yml

@@ -31,9 +31,9 @@ jobs:
         # zie ook https://www.postgresql.org/support/versioning/
         postgis: 
             # t/m november 2027
-            - 15-3.6-alpine
+            - 15-3.5-alpine
             # t/m november 2028
-            - 16-3.6-alpine
+            - 16-3.5-alpine
             # t/m november 2029
             - 17-3.6-alpine
             # t/m november 2030