[datadog_downtime_schedule] resource: Make monitor_identifier required

[l-teles]

This PR updates the datadog_downtime_schedule schema documentation, moving the monitor_identifier resource to required, since it's not optional:

* Failed to execute "terraform validate" in ../dummy/mycode
  ╷
  │ Error: Invalid Block
  │ 
  │   with datadog_downtime_schedule.i_want_to_go_down,
  │   on downtimes.tf line 7, in resource "datadog_downtime_schedule" "i_want_to_go_down":
  │    7: resource "datadog_downtime_schedule" "i_want_to_go_down" {
  │ 
  │ Block monitor_identifier must have a configuration value as the provider
  │ has marked it as required
  ╵

[l-teles]

All commits are signed now 👍

[l-teles]

Hey @maycmlee / @Supam,
It looks like something got messed up here. I've updated the branch with the latest changes from master and pushed the proposed changes again.
Can you please reapprove the PR (if needed) and trigger the necessary workflows to have this merged?

[maycmlee]

Hi @l-teles, my approval is still there, but reapproving just in case. I'm not familiar with Terraform releases, but it looks like you still need approval from API Reliability.

[Supam]

Hi there @l-teles
Looking at your pr, this will unfortunately not pass our CI as our documentation is generated using Terraform's tfplugindocs.
Furthermore this unwanted behaviour is a known bug form their tool: hashicorp/terraform-plugin-docs#363.
As far I understand, they do not have any plans on fixing the issue with how docs are generated for blocks and are recommending that people use nested attributes, which are only supported with protocol version 6.

The current workaround for this issue would be for us to disable the auto-generation for this resource and update the documentation by hand after. I will speak with the owning team internally if they would like to proceed with this or if they have something else planned.

Cheers