[Feature Request]: Fully deprecate the SMS Gateway app

### Preflight Checklist

- [x] I have read the [Contributing Guidelines](https://github.com/EnAccess/micropowermanager/blob/main/CONTRIBUTING.md) for this project, if it exists.
- [x] I agree to follow the [Code of Conduct](https://github.com/EnAccess/micropowermanager/blob/main/CODE_OF_CONDUCT.md) that this project adheres to.
- [x] I have searched the [issue tracker](https://github.com/EnAccess/micropowermanager/issues) for a feature request that matches the one I want to file, without success.

### Problem Description

Usage of the SMS Gateway App is deprecated in favour of using an SMS service provider like Africa's Talking.

### Proposed Solution

- [ ] Remove from the documentation
- [ ] Remove all SMS Gateway specific code from frontend
- [ ] Remove all SMS Gateway specific code from backend

### Alternatives Considered

N/A

### Additional Information

_No response_