Remove deprecated `is_read_only` reference in Datadog dashboards

**Action Required**
Before April 30, 2025, please migrate off of `is_read_only` to `restricted_roles` or consider participating in the beta for Restriction Policies.

Migrating to `restricted_roles` can be done independently.

If you’re managing Dashboards via Terraform:

[] Ensure that you’re using Datadog Terraform Provider v3.27.0 or higher
[] Remove is_read_only and restricted_role from your Dashboard Terraform resources
[] Create a new datadog_restriction_policy resource, referencing the respective Dashboard id and principals from the recently removed attributes
[] Run terraform apply

See [Datadog’s docs](https://docs.datadoghq.com/dashboards/guide/how-to-use-terraform-to-restrict-dashboard-edit/#restricting-a-dashboard-using-a-restriction-policy) for an example of what the Terraform resources look like in the end.