chore(flags): feature flag report contains feature info for all library versions (#35215)

This change goes together with
aws/aws-cdk-cli#782, to undo some decisions made in https://github.com/aws/aws-cdk/pull/35108/files and aws/aws-cdk-cli#742.

We used to store `unconfiguredBehavesLike` values for both `v1` and `v2` (and `v3`, `v4`, etc) into the feature flag report, but we only needed to store the value for the *current* library version. We are now defining the "v2" field to hold the current version information, and are updating the feature flag generation code to not emit `v1` data, and stay correct if we ever start releasing `v3`.

This is labeled as a `chore` and not a `fix` because it doesn't really fix user-visible behavior.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: rix0rrr

packages/aws-cdk-lib/core/lib/private/feature-flag-report.ts

@@ -14,7 +14,15 @@ function parseFeatureFlagInfo(flagName: string, info: FlagInfo, root: IConstruct
     userValue: userValue,
     recommendedValue: info.recommendedValue,
     explanation: info.summary,
-    unconfiguredBehavesLike: info.unconfiguredBehavesLike,
+
+    // This is a historical accident. We used to copy all `unconfiguredBehavesLike` data
+    // into the feature flag report, but we only needed to copy the behavior of the current library version.
+    // In order to not break existing reports, we keep the same structure, but we baptize the 'v2' field as
+    // the canonical name of the "current version" field, even for v3, v4, etc.
+    // It looks weird, but it's safe & backwards compatible.
+    unconfiguredBehavesLike: info.unconfiguredBehavesLike?.[feats.CURRENT_MV] ? {
+      v2: info.unconfiguredBehavesLike?.[feats.CURRENT_MV],
+    } : undefined,
   };
 
   return parsedFlag;