fix(bootstrap): bootstrap stack version was not bumped during previous update (#23669)

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This PR bumps the template version, since it was modified in #21975 but the version number was not changed.

cc: @rix0rrr who has assisted with merging the previous PR.

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Author: adrianmace

packages/aws-cdk/lib/api/bootstrap/bootstrap-template.yaml

@@ -587,7 +587,7 @@ Resources:
       Type: String
       Name:
         Fn::Sub: '/cdk-bootstrap/${Qualifier}/version'
-      Value: '14'
+      Value: '15'
 Outputs:
   BucketName:
     Description: The name of the S3 bucket owned by the CDK toolkit stack