[change:radius] Replace third-party JSONField with Django built-in JSONField

[Eeshu-Yadav]

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [x] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [ ] I have updated the documentation.

Reference to Existing Issue

Closes #600.

Description of Changes

This PR replaces the third-party jsonfield library with Django's built-in JSONField. The third-party JSONField library hasn't received updates in 5 years, while Django's built-in JSONField is now mature and actively maintained.

Changes Made:

• Replaced from jsonfield import JSONField with from django.db.models import JSONField in models
• Updated RadiusBatch.user_credentials and OrganizationRadiusSettings.sms_meta_data fields
• Removed "jsonfield~=3.1.0" dependency from setup.py
• Added migration 0041_replace_jsonfield_with_django_builtin.py to handle field type transition
• Updated test migrations to use Django's JSONField

Testing:

• All existing tests pass with the new implementation
• Verified RadiusBatch and SMS metadata functionality
• Migration tested without data loss

[Eeshu-Yadav]

@nemesifier kindly review

[coveralls]

coverage: 97.268% (-0.001%) from 97.269% when pulling ba82cd9b82d5320229abc568322f3a33963efbb1 on Eeshu-Yadav:issues/600-replace-jsonfield-clean into 38ee4426cee3a23a15c226c72c29bebce032fe9d on openwisp:master.

[Eeshu-Yadav]

@nemesifier kindly review