feat(ec2-alpha): ec2-alpha module is now in Developer Preview (#33230)

### Issue # (if applicable)

Tracking #[30762](#30762).

### Reason for this change

Moving alpha module for `VpcV2` construct to `developer-preview`.

### Description of changes

Set maturity to `developer-preview` and update the README stability banner for `ec2-alpha` module.

### 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)

----

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

packages/@aws-cdk/aws-ec2-alpha/README.md

@@ -3,19 +3,18 @@
 
 ---
 
-![cdk-constructs: Experimental](https://img.shields.io/badge/cdk--constructs-experimental-important.svg?style=for-the-badge)
+![cdk-constructs: Developer Preview](https://img.shields.io/badge/cdk--constructs-developer--preview-informational.svg?style=for-the-badge)
 
-> The APIs of higher level constructs in this module are experimental and under active development.
-> They are subject to non-backward compatible changes or removal in any future version. These are
-> not subject to the [Semantic Versioning](https://semver.org/) model and breaking changes will be
-> announced in the release notes. This means that while you may use them, you may need to update
-> your source code when upgrading to a newer version of this package.
+> The APIs of higher level constructs in this module are in **developer preview** before they
+> become stable. We will only make breaking changes to address unforeseen API issues. Therefore,
+> these APIs are not subject to [Semantic Versioning](https://semver.org/), and breaking changes
+> will be announced in release notes. This means that while you may use them, you may need to
+> update your source code when upgrading to a newer version of this package.
 
 ---
 
 <!--END STABILITY BANNER-->
 
-
 ## VpcV2
 
 `VpcV2` is a re-write of the [`ec2.Vpc`](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_ec2.Vpc.html) construct. This new construct enables higher level of customization

packages/@aws-cdk/aws-ec2-alpha/package.json

@@ -95,7 +95,7 @@
     "node": ">= 14.15.0"
   },
   "stability": "experimental",
-  "maturity": "experimental",
+  "maturity": "developer-preview",
   "awscdkio": {
     "announce": false
   },