[models] Replace thirdparty 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.
• [ ] 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 #1061.

Description of Changes

This PR replaces the third-party jsonfield package with Django's built-in JSONField across all OpenWISP controller models to modernize the codebase and remove dependency on an unmaintained package.

Changes Made:

• Updated 6 model files to use django.db.models.JSONField instead of third-party jsonfield.JSONField:

  • openwisp_controller/config/base/base.py
  • openwisp_controller/config/base/config.py
  • openwisp_controller/config/base/device_group.py
  • openwisp_controller/config/base/multitenancy.py
  • openwisp_controller/config/base/template.py
  • openwisp_controller/connection/base/models.py

• Created 2 migration files to safely alter field types:

  • openwisp_controller/config/migrations/0061_replace_jsonfield_with_django_builtin.py
  • openwisp_controller/connection/migrations/0010_replace_jsonfield_with_django_builtin.py

• Removed jsonfield-specific parameters (load_kwargs, dump_kwargs) that are not supported by Django's JSONField

• Fixed code quality issues by removing unused imports and duplicate imports

Technical Details:

• Django's JSONField handles JSON serialization internally without needing external parameters
• The load_kwargs={"object_pairs_hook": collections.OrderedDict} is no longer needed as Python 3.7+ dicts maintain insertion order
• The dump_kwargs={"indent": 4} formatting should be handled at the serialization/display layer, not the model level
• All existing JSON data remains compatible and accessible