cdk-build-tools: testFutureBehavior/testLegacyBehavior outdated for CDK v2

### Describe the bug

Methods `testFutureBehavior` and `testLegacyBehavior` were modified for CDK v2 development:
- `testFutureBehavior` ignores feature flags in CDK v2 because all feature flags are enabled by default
- Tests in `testLegacyBehavior` will be ignored because there is no legacy behavior when CDK v2 was released

This logic was correct during the development of CDK v2. Now, it must be possible to add new feature flags.

Instead of checking for version `2` in the following line, we need to change it to version `3`:
https://github.com/aws/aws-cdk/blob/2151a0e9b988723e050e6f37ed1780cced16c519/tools/%40aws-cdk/cdk-build-tools/lib/feature-flag.ts#L32

### Expected Behavior

- `testFutureBehavior` supports new feature flags
- `testLegacyBehavior` executes tests without feature flag

### Current Behavior

- `testFutureBehavior` doesn't support adding feature flags
- Tests in `testLegacyBehavior` will be skipped

### Reproduction Steps

Test cases in PR #18140 fail because `testFutureBehavior` doesn't execute the correct behavior after target branch of this PR was changed to v2 main branch.

### Possible Solution

Replace version `2` by `3` in the following lines:

https://github.com/aws/aws-cdk/blob/2151a0e9b988723e050e6f37ed1780cced16c519/tools/%40aws-cdk/cdk-build-tools/lib/feature-flag.ts#L32

https://github.com/aws/aws-cdk/blob/2151a0e9b988723e050e6f37ed1780cced16c519/tools/%40aws-cdk/cdk-build-tools/lib/feature-flag.ts#L60

### Additional Information/Context

_No response_

### CDK CLI Version

2.29.0

### Framework Version

_No response_

### Node.js Version

v16.15.1

### OS

MacOS

### Language

Typescript

### Language Version

_No response_

### Other information

I can create a PR to fix this issue. Please let me know if the proposed solution is correct.

Start in tools/@aws-cdk/cdk-build-tools/lib/feature-flag.ts at the version checks on lines 32 and 60. Review the test cases referenced from PR #18140, then verify that testFutureBehavior supports new feature flags and testLegacyBehavior executes tests without a feature flag.

Written by the indexing model from the issue text.