Make client_secret optional since it's not always required

[anderssonjohan]

Why:

Apps using the device flow does not have a client secret and can generate new access tokens using the refresh_token grant without this parameter in the request.

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

Example request:

requests.post(
    "https://github.com/login/oauth/access_token",
    data={
        "client_id": "<YOUR CLIENT ID>",
        "refresh_token": refresh_token,
        "grant_type": "refresh_token",
    },
)

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• For content changes, I have completed the self-review checklist.

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[github-actions]

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

---

Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
apps/creating-github-apps/authenticating-with-a-github-app/refreshing-user-access-tokens.md	fpt ghec ghes@ 3.12 3.11 3.10 3.9	fpt ghec ghes@ 3.12 3.11 3.10 3.9

---

fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[nguyenalex836]

@anderssonjohan Thanks so much for opening a PR! I'll get this triaged for review ✨

[skedwards88]

Thanks for this PR! I made a suggestion that I'll go ahead and incorporate, then we'll get this merged for you.

[github-actions]

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