What does this implement/fix? Explain your changes.

I was trying to create a comment under another comment (i.e. reply to a comment), but it looks like setting parent in createComment's input field does not work at all, the comment is posted directly under the post, not a response to another comment.
Changed the type of parent to Int and the issue was gone.
According to https://developer.wordpress.org/reference/functions/wp_new_comment/, when creating a new comment with parent ID, comment_parent field should be integer, not ID/string. Currently we're taking parent argument as ID.

Does this close any currently open issues?

No

Any relevant logs, error output, GraphiQL screenshots, etc?

There's no error message for this bug.
The following command would post the comment directly under post 3538 (parent does not work):

mutation CREATE_COMMENT {
  createComment(
    input: {commentOn: 3538 (Post ID), content: "This is a test comment, yo", author: "Jason", authorEmail: "xyz@gmail.com", parent: "Y29tbWVudDoxMDQ1"  (Parent Comment ID)}
  ) {
    success
  }
}

The following command would post a response under comment "Y29tbWVudDoxMDQ1" / 1045 after I changed type of parent to Int:

mutation CREATE_COMMENT {
  createComment(
    input: {commentOn: 3538 (Post ID), content: "This is a test comment, yo", author: "Jason", authorEmail: "xyz@gmail.com", parent: 1045  (Parent Comment ID)}
  ) {
    success
  }
}

Any other comments?

First time creating a PR to an open source project, please let me know if I need to change anything, thanks!

Where has this been tested?

Operating System: Windows 11

WordPress Version: 5.8.2