Mention Smee security caveat and alternatives on Testing webhooks - #46520

On December 2, 2026, GitHub will restrict `pull_request_target` on public repositories by default. To continue allowing the event trigger, configure an Actions policy. Learn more: https://gh.io/securely-using-pull_request_target#default-policy-for-pull_request_target

'base' input parameter is ignored when action is triggered by pull request event