Use Scheduler Group to isolate CloudWatch metrics

[ryamelv]

The project creates two Amazon EventBridge Schedules - one for the next event driven action and one for periodic backup checks.
An AWS CloudFormation bug currently prevents retrieving the ARN of a Schedule in a Schedule Group, so if the CDK app puts the Schedules in a Schedule Group the deployment fails.

• AWS::Scheduler::ScheduleGroup - Cannot create: Unable to retrieve Arn attribute for AWS::Scheduler::Schedule aws-cloudformation/cloudformation-coverage-roadmap#1726

Since the schedules are not in a schedule group, the CloudWatch Dashboard cannot isolate its metrics to only the stack's schedules - the related metrics show values for the "default" schedule group, which could contain schedules not related to the stack.

If/when the AWS CloudFormation bug is fixed, this project should move its schedules into a dedicated group.