nixpkgs branch protection rules: dismiss stale pull request approvals

[wolfgangwalther]

For NixOS/nixpkgs#415259, I would like to enable the following branch protection rule:

Dismiss stale pull request approvals when new commits are pushed
New, reviewable commits pushed will dismiss previous pull request review approvals.

This is an "additional setting" in the "Require a pull request before merging" option, which I think should be enabled already. It's available in both classic branch protection (which we're currently using) and the new rulesets. So this should be a simple additional checkmark only.

[wolfgangwalther]

Clarification: With NixOS/nixpkgs#415259 (review), this changed rule is not required, but a separate discussion on-top, aka "how should stale reviews count?".

[wolfgangwalther]

This hasn't come up again, so far. I think we're happy with how approvals are counted right now, so we don't need to enable this option.