chore(cdk): increase coverage limits (#31936)

### Issue

Relates to #31931

### Reason for this change

The AWS CLI currently requires comparatively low coverage by tests:
```
statements: 60
branches: 45
```

The actual coverage is thankfully higher:

```
=============================== Coverage summary ===============================
Statements   : 81.59% ( 5462/6694 )
Branches     : 68.05% ( 1764/2592 )
Functions    : 84.49% ( 1128/1335 )
Lines        : 81.9% ( 5211/6362 )
================================================================================
```

Make sure we don't drop below the current values.


### 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/jest.config.js

@@ -3,8 +3,13 @@ module.exports = {
     ...baseConfig,
     coverageThreshold: {
         global: {
-            statements: 60,
-            branches: 45,
+            // We want to improve our test coverage
+            // DO NOT LOWER THESE VALUES!
+            // If you need to break glass, open an issue to re-up the values with additional test coverage
+            statements: 81,
+            branches: 68,
+            functions: 84,
+            lines: 81
         },
     },