fix(ec2): passing keypair to instance unexpectedly does nothing (#28482)

Fixes by Specifying key pair reference in cfnInstance.

This will change behavior if both `keyName` and `keyPair` is set on an existing resource, since we will use `keyPair.keyPairName` instead of `keyName` now. However, there is no correct use case for specifying both `keyPair` and `keyName`, and given `keyName` is deprecated, this PR is introducing the correct behavior.

Closes #28478.

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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*

Author: ayush-shah-1501

packages/@aws-cdk-testing/framework-integ/test/aws-ec2/test/integ.key-pair.js.snapshot/KeyPairTestDefaultTestDeployAssertF103D058.assets.json

@@ -489,7 +489,7 @@
      "S3Bucket": {
       "Fn::Sub": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}"
      },
-     "S3Key": "a099fdfc61c84ffc56cef4fb2c9472483623ac865ce5d8fca88c89cf60d48d03.zip"
+     "S3Key": "4554b47be6f57b68c6c7a7391dcc73894866d2377fe174883351e7639097f292.zip"
     },
     "Timeout": 900,
     "MemorySize": 128,
@@ -588,6 +588,9 @@
      "Ref": "SsmParameterValueawsserviceamiamazonlinuxlatestal2023amikernel61x8664C96584B6F00A464EAD1953AFF4B05118Parameter"
     },
     "InstanceType": "t3.micro",
+    "KeyName": {
+     "Ref": "TestKeyPair38B6CD21"
+    },
     "SecurityGroupIds": [
      {
       "Fn::GetAtt": [