Update North Carolina SCCA age parameters to use proper unit type after core fix

Currently in PR #5575, we had to work around a parameter validation issue in policyengine-core by:

1. Changing to in the age group scale parameter
2. Hardcoding age limit values directly in the variables instead of using parameters

Once the policyengine-core issue is fixed (see PolicyEngine/policyengine-core#346), we should:

1. Update the parameter metadata to use the proper unit type ( or another appropriate unit)
2. Revert the hardcoded implementation to use the parameter-based approach
3. Ensure all tests pass with the parameter-based implementation

This will provide a cleaner, more maintainable implementation that properly leverages the parameter system.