fix(deps): upgrade djangorestframework to 3.18.1 for security fixes

Resolves medium severity Dependabot alerts: a potential bypass of
Django's DATA_UPLOAD_MAX_MEMORY_SIZE when parsing oversized JSON and
urlencoded bodies via DRF request.data, and disclosure of
GET-protected data by AdminRenderer when rendering invalid write
requests.

GHSA-2m8g-3cmr-wg3w, GHSA-g47c-3xmw-q6m2

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: matrixise

requirements/dev.txt

@@ -80,7 +80,7 @@ django-treebeard==4.8.0
     # via
     #   -c requirements/main.txt
     #   wagtail
-djangorestframework==3.17.1
+djangorestframework==3.18.1
     # via
     #   -c requirements/main.txt
     #   wagtail

requirements/main.txt

@@ -83,7 +83,7 @@ django-tasks==0.9.0
     #   wagtail
 django-treebeard==4.8.0
     # via wagtail
-djangorestframework==3.17.1
+djangorestframework==3.18.1
     # via wagtail
 draftjs-exporter==5.2.0
     # via wagtail