Migrate from Travis CI to GitHub Actions for CI/CD

**Description:**

We should migrate the EvalAI CI/CD pipeline from **Travis CI** to **GitHub Actions**, which is now the more widely adopted and integrated CI solution for GitHub repositories and is free.

A draft migration was already initiated in [[PR #4576](https://github.com/Cloud-CV/EvalAI/pull/4576)](https://github.com/Cloud-CV/EvalAI/pull/4576), which includes the basic setup of GitHub Actions workflows for backend testing.

**Motivation:**

* Travis CI has reduced OSS support and slower performance.
* GitHub Actions provides better integration, caching, and visibility.
* Unifying CI workflows with the GitHub ecosystem will simplify development and collaboration.

**Tasks:**

* [ ] Finalize and merge [[PR #4576](https://github.com/Cloud-CV/EvalAI/pull/4576)](https://github.com/Cloud-CV/EvalAI/pull/4576) with necessary fixes and enhancements.
* [ ] Ensure the backend test suite runs reliably on GitHub Actions (including database services, coverage, etc.).
* [ ] Set up CI checks for linting, migrations, and static type checks (if any).
* [ ] The deployment runs from the Github CI.
* [ ] Decommission `.travis.yml` and remove Travis-specific config/integration from the repository.
* [ ] Update documentation to reflect the CI migration (e.g., in `README.md` or `CONTRIBUTING.md`).

**Acceptance Criteria:**

* GitHub Actions fully replaces Travis CI for all pull requests and merges.
* All tests and jobs pass successfully and consistently on GitHub Actions.
* Travis CI integration is removed both from the repo and any external services.

**References:**

* Draft PR for GitHub Actions setup: [[#4576](https://github.com/Cloud-CV/EvalAI/pull/4576)](https://github.com/Cloud-CV/EvalAI/pull/4576)