fix(ecs): enableDeploymentAlarms should mutate the alarmNames property

[vito-laurenza-zocdoc]

Issue #36308

Closes #36308

Reason for this change

Fixes a silent bug in the ecs module.

Description of changes

The code is mistakenly using concat() which returns a new array rather than modifying the existing one (which was the original intent of the code).

Describe any new or updated permissions being added

n/a

Description of how you validated changes

test case added

Checklist

• [x] My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

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

[github-actions[bot]]

	Tests	Passed ☑️	Skipped	Failed ❌️
Security Guardian Results	26 ran	24 passed		2 failed

Test	Result
Security Guardian Results
packages/@aws-cdk-testing/framework-integ/test/aws-ecs/test/base/integ.blue-green-deployment-strategy.js.snapshot/aws-ecs-blue-green-deployment.template.json
ec2-no-open-security-groups.guard	❌ failure
iam-no-wildcard-actions-inline.guard	❌ failure

• View Security Guardian Results

[github-actions[bot]]

	Tests	Passed ☑️	Skipped	Failed ❌️
Security Guardian Results with resolved templates	26 ran	24 passed		2 failed

Test	Result
Security Guardian Results with resolved templates
packages/@aws-cdk-testing/framework-integ/test/aws-ecs/test/base/integ.blue-green-deployment-strategy.js.snapshot/aws-ecs-blue-green-deployment.template.json
ec2-no-open-security-groups.guard	❌ failure
iam-no-wildcard-actions-inline.guard	❌ failure

• View Security Guardian Results with resolved templates

[pahud]

This PR is currently blocked by this:

❌ Fixes must contain a change to an integration test file and the resulting snapshot.

You will need to add/update integ test to unblock this or if you think integ tests are not necessary, please follow the suggestion https://github.com/aws/aws-cdk/pull/36309#pullrequestreview-3545387881

[vito-laurenza-zocdoc]

This PR is currently blocked by this:

❌ Fixes must contain a change to an integration test file and the resulting snapshot.

You will need to add/update integ test to unblock this or if you think integ tests are not necessary, please follow the suggestion #36309 (review)

Thanks. I've added basic coverage for this case in the existing integration test as it was not exercised there before.

[vito-laurenza-zocdoc]

@pahud can you advise about these security guardian failures?