Update prop value to use constants

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Let's move the prop value for the commit-message into a constant.js and use that constant instead. Second, let's update the respective specs to use that constants.

<upload-blob-modal
  :commit-message="__('Upload New File')" // Change to constant
/>
<new-directory-modal
  :commit-message="__('Add new directory')" // Change to constant
/>

More Info

The following discussion from !71154 (merged) should be addressed:

Edited by 🤖 GitLab Bot 🤖