chore(core): fix bootstrapQualifier (aws#23246)

----

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### Adding new Construct Runtime Dependencies:

* [ ] This PR adds new construct runtime dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-construct-runtime-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

packages/@aws-cdk/core/lib/stack-synthesizers/nested.ts

@@ -11,13 +11,12 @@ import { IStackSynthesizer, ISynthesisSession } from './types';
  * App builder do not need to use this class directly.
  */
 export class NestedStackSynthesizer extends StackSynthesizer {
-  /**
-   * The qualifier used to bootstrap this stack
-   */
-  public readonly bootstrapQualifier?: string;
   constructor(private readonly parentDeployment: IStackSynthesizer) {
     super();
-    this.bootstrapQualifier = parentDeployment.bootstrapQualifier;
+  }
+
+  public get bootstrapQualifier(): string | undefined {
+    return this.parentDeployment.bootstrapQualifier;
   }
 
   public addFileAsset(asset: FileAssetSource): FileAssetLocation {

packages/@aws-cdk/core/lib/stack-synthesizers/stack-synthesizer.ts

@@ -19,6 +19,14 @@ import { IStackSynthesizer, ISynthesisSession } from './types';
  * and could not be accessed by external implementors.
  */
 export abstract class StackSynthesizer implements IStackSynthesizer {
+
+  /**
+   * The qualifier used to bootstrap this stack
+   */
+  public get bootstrapQualifier(): string | undefined {
+    return undefined;
+  }
+
   private _boundStack?: Stack;
 
   /**