Update prop value to use constants
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=342630)
</details>
<!--IssueSummary end-->
Let's move the prop value for the `commit-message` into a [constant.js](https://gitlab.com/gitlab-org/gitlab/-/blob/ae2613fdc24610fff8093decece2bba90ee5ab87/app/assets/javascripts/repository/constants.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 should be addressed:
- [ ] @jerasmus started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/71154#note_693502133): (+1 comment)
> **suggestion:** WDYT of adding this in our [constants.js](https://gitlab.com/gitlab-org/gitlab/-/blob/ae2613fdc24610fff8093decece2bba90ee5ab87/app/assets/javascripts/repository/constants.js) file? We could also re-use it in our specs.
issue