ci: increase stale action operations-per-run to 500 (#4312)

* increase stale action operations-per-run to 500

Assisted-by: Claude Sonnet 4.6

* add changelog entry for stale ops-per-run increase

Assisted-by: Claude Sonnet 4.6

* add TODO to revert once backlog is cleaned up

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>