fix(cloudformation-diff): cdk diff fails on node14 (#28067)

Some folks are still stuck on node14. Replacing `.at()` with conventional array access to make it easier for them. In this particular instance there's really no benefit to the code using it.

But seriously, y'all should upgrade to a current, supported version like Node.js 18.

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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/cloudformation-diff/lib/diff/util.ts

@@ -184,5 +184,5 @@ function database(): SpecDatabase {
  * The database is loaded lazily and cached across multiple calls to `loadResourceModel`.
  */
 export function loadResourceModel(type: string): Resource | undefined {
-  return database().lookup('resource', 'cloudFormationType', 'equals', type).at(0);
+  return database().lookup('resource', 'cloudFormationType', 'equals', type)[0];
 }