Remove deprecated `is_read_only` reference in Datadog dashboards

[mdeggies]

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 for an example of what the Terraform resources look like in the end.