feat(ec2): add c6i instances (#17237)

New C6I instances just got released:

https://aws.amazon.com/blogs/aws/new-amazon-ec2-c6i-instances-powered-by-the-latest-generation-intel-xeon-scalable-processors/

Docs have already been updated:
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-instance.html#cfn-ec2-instance-instancetype

<img width="1078" alt="Screen Shot 2021-10-29 at 3 11 00 PM" src="https://user-images.githubusercontent.com/31543/139502936-a34cd4f0-0d8e-4f2e-a178-078aab7d00c4.png">


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

packages/@aws-cdk/aws-ec2/lib/instance-types.ts

@@ -208,6 +208,16 @@ export enum InstanceClass {
    */
   C5 = 'c5',
 
+  /**
+   * Compute optimized instances, 6th generation
+   */
+  COMPUTE6_INTEL = 'c6i',
+
+  /**
+   * Compute optimized instances, 6th generation
+   */
+  C6I = 'c6i',
+
   /**
    * Compute optimized instances with local NVME drive, 5th generation
    */