chore: Update example dependencies

[lantoli]

Proposed changes

Update example dependencies

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 updates client-side example dependencies in the MERN CDK CI/CD project.

• Bumps react-router-dom from v7.x to v6.8.0
• Adds several package overrides (e.g., ws, webpack-dev-server, postcss, etc.)

Comments suppressed due to low confidence (1)

examples/mern-cdk-ci-cd/mern/client/package.json:17

• [nitpick] The overrides field is only supported in npm v8.3+ (or Yarn). Please add a note in README or TESTING.md specifying the required package manager and version to ensure these overrides are applied.

"overrides": {