chore(events): rule cannot have more than 5 targets (#30470)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change
This is a minor refactor for preventing an Event Rule to have at most 5 associated targets defined during a synth. This is the main limit set by EventBridge https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-quota.html#eb-limits



### Description of changes
This is a very simple change in `aws-events` module to check if the `targets` array length of an `Event` is greater than 5. If it is then it throws an Error during build time.



### Description of how you validated changes
Added a unit test by in `rule.test.ts` by verifying an Error is being thrown when a Rule has more than 5 associated targets.



### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk-lib/aws-events/lib/rule.ts

@@ -314,6 +314,10 @@ export class Rule extends Resource implements IRule {
       errors.push('Either \'eventPattern\' or \'schedule\' must be defined');
     }
 
+    if (this.targets.length > 5) {
+      errors.push('Event rule cannot have more than 5 targets.');
+    }
+
     return errors;
   }
 

packages/aws-cdk-lib/aws-events/test/rule.test.ts

@@ -72,6 +72,25 @@ describe('rule', () => {
     });
   });
 
+  test('rule cannot have more than 5 targets', () => {
+    const app = new cdk.App();
+    const stack = new cdk.Stack(app);
+    const resource = new Construct(stack, 'Resource');
+    const rule = new Rule(stack, 'MyRule', {
+      schedule: Schedule.rate(cdk.Duration.minutes(10)),
+      targets: [
+        new SomeTarget('T1', resource),
+        new SomeTarget('T2', resource),
+        new SomeTarget('T3', resource),
+        new SomeTarget('T4', resource),
+        new SomeTarget('T5', resource),
+        new SomeTarget('T6', resource),
+      ],
+    });
+
+    expect(() => app.synth()).toThrow(/Event rule cannot have more than 5 targets./);
+  });
+
   test('get rate as token', () => {
     const app = new cdk.App();
     const stack = new cdk.Stack(app, 'MyScheduledStack');