(flags): ability to disable feature flags are not configured warning

### Describe the feature

I'd like to have an option to disable the "feature flags are not configured" warning

### Use Case

It's annoying and just pollutes the output.

### Proposed Solution

Have an option in `cdk.json` to disable it.

### Other Information

When managing projects via projen, feature flags are kinda clunky and by default are all off. I cannot enable them all, as it's a breaking behaviour.

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.1029.2

### Environment details (OS name and version, etc.)

macOS