Document that clientId may be assigned to the appId property

[joebowbeer]

There are two places in the README that should mention that the clientId string can be assigned to the appId property, whose type is now number | string. In fact, providing the clientId string is now recommended.

1. https://github.com/octokit/auth-app.js/tree/main?tab=readme-ov-file#createappauthoptions-or-new-octokit-auth-
2. https://github.com/octokit/auth-app.js/tree/main?tab=readme-ov-file#json-web-token-jwt-authentication-1

Originally posted by @gr2m in #606 (comment)

[github-actions]

👋 Hi! Thank you for this contribution! Just to let you know, our GitHub SDK team does a round of issue and PR reviews twice a week, every Monday and Friday! We have a process in place for prioritizing and responding to your input. Because you are a part of this community please feel free to comment, add to, or pick up any issues/PRs that are labeled with Status: Up for grabs. You & others like you are the reason all of this works! So thank you & happy coding! 🚀