Removing deprecated '@ember/polyfills'

[marcemira]

This pull request refactors the start-app.js test helper to remove the deprecated @ember/polyfills import and replace the merge function with the ES6 spread operator for merging objects.

Changes Made:

• Removed the import statement for merge from @ember/polyfills.
• Updated the merge function call to use the ES6 spread operator for merging default attributes with provided attributes.

Reason for Change:

• The @ember/polyfills package has been deprecated, and using the ES6 spread operator is a modern and more efficient way to handle object merging. This change ensures compatibility with future Ember.js versions and removes reliance on deprecated functionality.

Impact:

• No functionality changes.
• Improved code readability.
• Ensures compatibility with newer Ember.js versions.

[bvedad]

What is the status of this MR? It is blocking our Ember v5 upgrade.