chore: update CHANGELOG.v2.alpha.md to announce kinesisfirehose as developer preview (#31969)

Update the Change log file to announce kinesisfirehose as developer preview

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.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: moelasmar

CHANGELOG.v2.alpha.md

@@ -17,6 +17,7 @@ All notable changes to this project will be documented in this file. See [standa
 * **pipes-enrichments:** support API Gateway enrichment ([#31794](https://github.com/aws/aws-cdk/issues/31794)) ([09052c2](https://github.com/aws/aws-cdk/commit/09052c2060c410028896fd54e76a857b2141c8a4)), closes [#29384](https://github.com/aws/aws-cdk/issues/29384)
 * **pipes-targets:** add SageMaker ([#30696](https://github.com/aws/aws-cdk/issues/30696)) ([a5fdf57](https://github.com/aws/aws-cdk/commit/a5fdf570beb1456b1307276f56d90fd1ba0b46d8))
 * **redshift-alpha:** query execution timeout setting during table creation ([#31818](https://github.com/aws/aws-cdk/issues/31818)) ([40f07ae](https://github.com/aws/aws-cdk/commit/40f07ae330d074cfa7861e24a0427da7ec427f68)), closes [#31329](https://github.com/aws/aws-cdk/issues/31329)
+* **kinesisfirehose-alpha:** kinesis firehose and kinesis firehose destinations modules are now in Developer Preview ([#31952](https://github.com/aws/aws-cdk/pull/31952))
 
 ### Bug Fixes