build: Change Django version constraint to <6.0

[feanil]

Updated the django version constraint to allow all 5.x updates since we've now running this on all released services.

Merge checklist:

• All reviewers approved
• CI build is green
• If adding new checks, followed how_tos/0001-adding-new-checks.rst
• Changelog record added (if needed)
• Documentation updated (not only docstrings) (if needed)
• Commits are squashed