chore(core): can't override bootstrapQualifier property (#23235)

On newer versions of typescript you will get an error like `'bootstrapQualifier' is defined as a property in class 'StackSynthesizer', but is overridden here in 'DefaultStackSynthesizer' as an accessor.`

Just make the property mutable instead.


----

### All Submissions:

* [ ] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### 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*

Author: corymhall

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

@@ -117,6 +117,9 @@ export class CliCredentialsStackSynthesizer extends StackSynthesizer {
     }
   }
 
+  /**
+   * The qualifier used to bootstrap this stack
+   */
   public get bootstrapQualifier(): string | undefined {
     return this.qualifier;
   }

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

@@ -324,6 +324,9 @@ export class DefaultStackSynthesizer extends StackSynthesizer {
     }
   }
 
+  /**
+   * The qualifier used to bootstrap this stack
+   */
   public get bootstrapQualifier(): string | undefined {
     return this.qualifier;
   }

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

@@ -11,6 +11,9 @@ 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();

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

@@ -19,8 +19,6 @@ import { IStackSynthesizer, ISynthesisSession } from './types';
  * and could not be accessed by external implementors.
  */
 export abstract class StackSynthesizer implements IStackSynthesizer {
-  public readonly bootstrapQualifier?: string;
-
   private _boundStack?: Stack;
 
   /**