[integration key] do not error with deprecation on output-only usage

[drew-u410]

Fixes the integration_key deprecation warning when only used on output and not input.

Detail

• In [CSGI-2268 ] Deprecate integration_key attribute mutation for Service Integrations #775, the integration_key was marked as deprecated with the intent for input (makes sense).
• Post-shipping, it was found that this when used on output would still provide the deprecation message which is not expected or desired (example: documentation pagerduty_service_integration integration_key attribute is deprecated #782).
• By removing Optional: true, this should make it output-only in Terraform, to my knowledge.

Testing

• Tests pass, happy to do other things if desired.

go test -timeout=30s -parallel=4 github.com/terraform-providers/terraform-provider-pagerduty github.com/terraform-providers/terraform-provider-pagerduty/pagerduty
?   	github.com/terraform-providers/terraform-provider-pagerduty	[no test files]
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	4.227s

[imjaroiswebdev]

Thanks @drew-u410. The issue was addressed in #801. However, this and any other contributions from you are very welcomed.