Add information about what happens with Gists after a username change

[billythekid]

Code of Conduct

• [X] I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/account-and-profile/setting-up-and-managing-your-personal-account-on-github/managing-personal-account-settings/changing-your-github-username

What part(s) of the article would you like to see updated?

Add a small section explaining the impact on Gists (PR incoming for this)

Additional information

No response

[welcome[bot]]

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[cmwilson21]

@billythekid Thank you for opening an issue and linking it to your PR! ✨