Replace toggle with checkbox in upload/delete file, create dir modals

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What does this MR do and why?

To match the Pajamas guidelines in the Repository page, in upload file and create directory modals 'create a new MR' toggle component is replaced by the checkbox.

Closes: #479249 (closed)

MR acceptance checklist

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Screenshots or screen recordings

Before	After
before.mp4	upload-file-after.mp4 create-dir-after.mp4

How to set up and validate locally

1. Navigate to the Code -> Repository page.
2. Click on the +.
3. Click on the Upload file, New directory. In case of file upload change the default branch name in the corresponding input.
4. To verify a delete file modal. Upload any file to the repo -> click on the uploaded file -> press the Delete button -> change the default branch name in the corresponding input.