chore: Cleanup unmaintained examples

[lantoli]

Proposed changes

Cleanup unmaintained examples: mern-cdk-ci-cd.

Jira ticket: CLOUDP-342275

Please include a summary of the fix/feature/change, including any relevant motivation and context.

Link to any related issue(s):

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as
  expected)
• This change requires a documentation update

Required Checklist:

• I have signed the MongoDB CLA
• I have added tests that prove my fix is effective or that my feature works
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• I have tested the CDK constructor in a CFN stack. See TESTING.md
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

[Copilot]

Pull Request Overview

This PR removes an unmaintained MERN stack example that included a complete CI/CD pipeline with MongoDB Atlas integration. The example demonstrated full-stack development with React frontend, Express backend, and automated testing, but was being cleaned up as part of maintenance activities.

• Removes the entire mern-cdk-ci-cd example directory structure
• Eliminates associated CI/CD pipeline configurations and AWS CDK infrastructure code
• Cleans up test files, build configurations, and documentation

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[AgustinBettati]

LGTM

[oarbusi]

Nice!