Upgrade the testing versions of DRF to 3.14, 3.15

Author: tim-schilling

.github/workflows/testing.yml

@@ -32,7 +32,7 @@ jobs:
       matrix:
         python-version: [ "3.9", "3.10", "3.11", "3.12", "3.13"]
         django-version: [ "4.2", "5.0", "5.1"]
-        drf-version: [ "3.11", "3.12", "3.13" ]
+        drf-version: ["3.14", "3.15"]
         exclude:
           # Python 3.9 is incompatible with Django v5+
           - django-version: 5.0