Remove `"COMMENT"` as the default event for `create_review` (#3078)

Fixes a discrepancy between PyGithub for creating a pull request review,
compared to the Github API. When using the REST API, not specifying an
`event` defaults to using `PENDING` mode, which creates a review and
automatically adds the user as a reviewer. In the current PyGithub
implementation, if no `event` parameter is given, it defaults to
`COMMENT`, which does not add the user as a reviewer. It is not possible
to use `event="PENDING"` via the API, it's just the default Github
creates server side when `event` is sent empty.


https://docs.github.com/en/rest/pulls/reviews?apiVersion=2022-11-28#create-a-review-for-a-pull-request

**Relevant section pasted below:**

`event` string
The review action you want to perform. The review actions include:
`APPROVE`, `REQUEST_CHANGES`, or `COMMENT`. **By leaving this blank, you
set the review action state to `PENDING`**, which means you will need to
[submit the pull request
review](https://docs.github.com/rest/pulls/reviews#submit-a-review-for-a-pull-request)
when you are ready.

Can be one of: APPROVE, REQUEST_CHANGES, COMMENT **<- note `PENDING`
can't be selected directly, only indirectly by leaving `event` blank,
and thus setting the default to `COMMENT` in PyGithub this incorrectly
blocks this**

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Co-authored-by: Enrico Minack <github@enrico.minack.dev>