Configure pull request limit for users without write access

The [open pull request count](https://github.com/nodejs/node/pulls?q=sort%3Aupdated-desc+is%3Apr+state%3Aopen+) on `nodejs/node` has grown beyond 1000+. A portion of these are from individual contributors who have a high number of open PRs at the same time, which places demands on finite collaborator review capacity. GitHub provides a setting to [limit the number of open pull requests](https://docs.github.com/en/communities/moderating-comments-and-conversations/limiting-interactions-in-your-repository#configuring-the-pull-request-limit) from users without write access. This helps balance the need for open contribution with the reality of limited human review resources. ### Proposal **Should we set a pull request limit for users without write access?** This idea was discussed among collaborators, and **5** was suggested as a reasonable starting point. We'd like to hear broader community input before making a decision. ### Rationale - Every open PR requires collaborator time for review, feedback, and follow-up. A high volume of open PRs from a single contributor can reduce the time available to review contributions from others. - With the rise of AI-assisted tooling, it is now easier than ever to generate pull requests at a higher frequency, which can outpace the capacity of human reviewers. - A limit encourages contributors to prioritize their most impactful changes and work with collaborators to get existing PRs merged or closed before opening new ones. - It also serves as a backstop against automated spamming attacks that open multiple PRs. - Contributors who are consistently engaged at a level that requires more open PRs would be candidates for collaborator status, which is not subject to the limit. ### Notes - The limit applies only to users **without write access**. Collaborators are not affected. - We can start with 5 (or some other number based on consensus) and re-evaluate based on feedback. - GitHub also provides a "bypass list" but we propose not using it to avoid the overhead of managing additions and removals.