fix(core): feature flag values should be booleans (#21759)

`FeatureFlags.isEnabled()` returns `boolean | undefined`, but if the
value of the feature flag is something other than a boolean, it will
actually return that type instead. For example,

```ts
node.setContext(someFeatureFlag, 'true');
const enabled = FeatureFlags.of(node).isEnabled(someFeatureFlag);
if (enabled) {
  // will work
}

if (enabled === true) {
  // will not work because enabled === 'true'
}
```

It looks like the only place this bug crops up is in the
`FunctionVersionUpgrade` aspect.

https://github.com/aws/aws-cdk/blob/8b0832334afee496355e7aeb684773d8f939f058/packages/@aws-cdk/aws-lambda/lib/function.ts#L1306


----

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### Adding new Unconventional Dependencies:

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### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
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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*

Author: corymhall

packages/@aws-cdk/core/lib/feature-flags.ts

@@ -32,6 +32,6 @@ export class FeatureFlags {
       }
       return true;
     }
-    return context ?? cxapi.futureFlagDefault(featureFlag);
+    return context !== undefined ? Boolean(context) : cxapi.futureFlagDefault(featureFlag);
   }
 }

packages/@aws-cdk/core/test/feature-flags.test.ts

@@ -23,5 +23,13 @@ describe('feature flags', () => {
 
       expect(FeatureFlags.of(stack).isEnabled('non-existent-flag')).toEqual(false);
     });
+
+    test('strings are evaluated as boolean', () => {
+      const stack = new Stack();
+      stack.node.setContext(cxapi.NEW_STYLE_STACK_SYNTHESIS_CONTEXT, 'true');
+
+      const actual = FeatureFlags.of(stack).isEnabled(cxapi.NEW_STYLE_STACK_SYNTHESIS_CONTEXT);
+      expect(actual).toEqual(true);
+    });
   });
 });