make display name optional on monitoring notification channel

[megan07]

I could go either way on this, but to stay consistent with the API, I made it Optional. If we think it should be Required (since that is the recommendation of the API), I'm happy to just make a doc update.

Fixes hashicorp/terraform-provider-google#6084

Release Note Template for Downstream PRs (will be copied)

monitoring: made `display_name` optional on `google_monitoring_notification_channel `

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 9 insertions(+), 9 deletions(-))
Terraform Beta: Diff ( 2 files changed, 9 insertions(+), 9 deletions(-))