fix: upgrade numpy version to resolve cython build failure

[samay871]

Description

Fixed the Docker build failure caused by the incompatibility between the existing NumPy version and newer Cython releases.

Problem

The development environment setup was failing during the dependency installation step while building the Django and Celery Docker images.

The issue was caused by the outdated NumPy dependency:
numpy==1.18.4

This version allows incompatible Cython versions to be installed during dependency resolution, which results in build failures.

Solution

Updated NumPy from:
numpy==1.18.4

to:
numpy==1.21.6

NumPy 1.21.x includes the required Cython version constraint, resolving the dependency conflict without requiring any Dockerfile changes or additional pip flags.

Testing

• Rebuilt Django Docker image successfully.
• Rebuilt Celery Docker image successfully.
• Verified that the development containers build and start correctly using docker-compose.dev.yml.

Files Changed

• esim-cloud-backend/requirements.txt