chore(migrate): improve warning (#28025)

This feature is still under development and the warning isn't clear enough on that point.

I also added in a small improvement to compare the input language as lowercase. No need to throw an error just because someone types TypeScript instead of typescript.



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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*

packages/aws-cdk/lib/cdk-toolkit.ts

@@ -703,8 +703,8 @@ export class CdkToolkit {
    * @param options Options for CDK app creation
    */
   public async migrate(options: MigrateOptions): Promise<void> {
-    warning('This is an experimental feature. We make no guarantees about the outcome or stability of the functionality.');
-    const language = options.language ?? 'typescript';
+    warning('This is an experimental feature and development on it is still in progress. We make no guarantees about the outcome or stability of the functionality.');
+    const language = options.language?.toLowerCase() ?? 'typescript';
 
     try {
       validateSourceOptions(options.fromPath, options.fromStack);