chore(docs): improve and clean up CONTRIBUTING for feature flags (#18485)

Make both sets of helper methods (`FeatureFlags.of(..).isEnabled` and
`testFutureBehavior`/`testLegacyBehavior`) more prominent, and clean up the
language around v2 and flipping behavior there, as that's no longer relevant.


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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: njlynch

CONTRIBUTING.md

@@ -803,35 +803,17 @@ The pattern is simple:
    with the name of the context key that **enables** this new feature (for
    example, `ENABLE_STACK_NAME_DUPLICATES`). The context key should be in the
    form `module.Type:feature` (e.g. `@aws-cdk/core:enableStackNameDuplicates`).
-2. Use `node.tryGetContext(cxapi.ENABLE_XXX)` to check if this feature is enabled
+2. Use `FeatureFlags.of(construct).isEnabled(cxapi.ENABLE_XXX)` to check if this feature is enabled
    in your code. If it is not defined, revert to the legacy behavior.
 3. Add your feature flag to the `FUTURE_FLAGS` map in
    [cx-api/lib/features.ts](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/cx-api/lib/features.ts).
    This map is inserted to generated `cdk.json` files for new projects created
    through `cdk init`.
-4. In your PR title (which goes into CHANGELOG), add a `(under feature flag)` suffix. e.g:
+4. In your tests, use the `testFutureBehavior` and `testLegacyBehavior` [jest helper methods] to test the enabled and disabled behavior.
+5. In your PR title (which goes into CHANGELOG), add a `(under feature flag)` suffix. e.g:
 
     `fix(core): impossible to use the same physical stack name for two stacks (under feature flag)`
 
-In the [next major version of the
-CDK](https://github.com/aws/aws-cdk/issues/3398) we will either remove the
-legacy behavior or flip the logic for all these features and then
-reset the `FEATURE_FLAGS` map for the next cycle.
-
-### Feature Flags - CDKv2
-
-We have started working on the next version of the CDK, specifically CDKv2. This is currently being maintained
-on a separate branch `v2-main` whereas `master` continues to track versions `1.x`.
-
-Feature flags introduced in the CDK 1.x and removed in 2.x, must be added to the `FUTURE_FLAGS_EXPIRED` list in
-[cx-api/lib/features.ts](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/cx-api/lib/features.ts)
-on the `v2-main` branch.
-This will make the default behaviour in CDKv2 as if the flag is enabled and also prevents users from disabling
-the feature flag.
-
-A couple of [jest helper methods] are available for use with unit tests. These help run unit tests that test
-behaviour when flags are enabled or disabled in the two major versions.
-
 [jest helper methods]: https://github.com/aws/aws-cdk/blob/master/tools/@aws-cdk/cdk-build-tools/lib/feature-flag.ts
 
 ## Versioning and Release