Add alert_trigger_preferences to the notification policy filter block.

[Nmishin]

Closes: #2082

[github-actions]

changelog detected ✅

[jasoncabot]

Change looks good to me. For future reference you can get the filters available from the API with a valid token / account:

curl -X GET "https://api.cloudflare.com/client/v4/accounts/$ACCOUNT/alerting/v3/available_alerts" \
     -H "Authorization: Bearer $TOKEN" \
     -H "Content-Type:application/json"

[github-actions]

This functionality has been released in v4.9.0 of the Terraform Cloudflare Provider.

Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!