set django-cors-headers to <3.12.0

3.12.0 brings django > 3.2 which eoxserver does not yet support

requirements.txt

@@ -5,7 +5,7 @@ click<9.0
 pyyaml<6.0
 jsonschema<5.0
 boto3<2.0
-django-cors-headers<4.0
+django-cors-headers<3.12.0
 django-structlog==2.2.0
 pystac<2.0
 http-access==0.1.3