Update the stale PR bot message

[mjwolf]

Add a friendlier stale PR message, based from the Beats stale message.

This will hopefully also prompt contributors to respond, so we'll be better able to track PRs people are still interested in contributing.

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• Have you signed the contributor license agreement? y
• Have you followed the contributor guidelines? y
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? n/a
• If submitting code/script changes, have you verified all tests pass locally using make test? n/a
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? n/a
• Is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed. y
• Have you added an entry to the CHANGELOG.next.md? n/a