[change] Migrate autocomplete filters to a well maintained dependency

Summary

Migrate away from django-admin-autocomplete-filter to a maintained and license-compatible alternative.

Context

django-admin-autocomplete-filter is currently used in openwisp-utils but:

• The project is unmaintained.
• Its license is incompatible with openwisp-utils licensing requirements.

Proposed Change

Replace django-admin-autocomplete-filter with a maintained alternative.

Preferred candidate:

• https://github.com/vigo/django-admin-list-filter

Reasons:

• Actively maintained.
• MIT licensed.
• Uses Django native admin autocomplete infrastructure.
• Minimal scope and close functional match for current usage.

Acceptance Criteria

• No dependency on django-admin-autocomplete-filter.
• Feature parity for existing admin autocomplete filters.
• Compatibility with supported Django versions.
• No license incompatibilities introduced.
• Test the results on main modules like openwisp-controller: we need to ensure the admin UI works and looks fine.

Notes

Other alternatives were evaluated, but django-admin-list-filter offers the best balance of maintenance status, licensing, and minimal integration effort. Feel free to propose other alternatives if you think they work better.