Add a stale ticket tracker for github PRs

[KevinOConnor]

This change will mark any PR that hasn't been updated in 60 days with a stale label. If there are no updates in 7 days after that, the PR will be closed.

There's quite a few open PRs right now - many of them are relevant and should be looked at. However, quite a few are just stale and are unlikely to be followed up on. Marking them as stale may help reduce the list of open tickets which could hopefully make it easier to identify which PRs need more attention.

-Kevin

[github-actions[bot]]

Thank you for your contribution to Klipper. Unfortunately, a reviewer has not assigned themselves to this GitHub Pull Request. All Pull Requests are reviewed before merging, and a reviewer will need to volunteer. Further information is available at: https://www.klipper3d.org/CONTRIBUTING.html

There are some steps that you can take now:

1. Perform a self-review of your Pull Request by following the steps at: https://www.klipper3d.org/CONTRIBUTING.html#what-to-expect-in-a-review If you have completed a self-review, be sure to state the results of that self-review explicitly in the Pull Request comments. A reviewer is more likely to participate if the bulk of a review has already been completed.
2. Consider opening a topic on the Klipper Discourse server to discuss this work. The Discourse server is a good place to discuss development ideas and to engage users interested in testing. Reviewers are more likely to prioritize Pull Requests with an active community of users.
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Unfortunately, if a reviewer does not assign themselves to this GitHub Pull Request then it will be automatically closed. If this happens, then it is a good idea to move further discussion to the Klipper Discourse server. Reviewers can reach out on that forum to let you know if they are interested and when they are available.

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