fix(cli): downgrade bootstrap stack error message needs a hint for ne…

…w-style synthesis (aws#16237)

Fix to the bootstrap error message on the version comparison to provide to peek at solution so as to warn users about new style stack synthesis not being enabled automatically in the cdk.json as the potential root cause

Fixes aws#16009

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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/api/bootstrap/deploy-bootstrap.ts

@@ -65,7 +65,7 @@ export class BootstrapStack {
 
     const newVersion = bootstrapVersionFromTemplate(template);
     if (this.currentToolkitInfo.found && newVersion < this.currentToolkitInfo.version && !options.force) {
-      throw new Error(`Not downgrading existing bootstrap stack from version '${this.currentToolkitInfo.version}' to version '${newVersion}'. Use --force to force.`);
+      throw new Error(`Not downgrading existing bootstrap stack from version '${this.currentToolkitInfo.version}' to version '${newVersion}'. Use --force to force or set the '@aws-cdk/core:newStyleStackSynthesis' feature flag in cdk.json to use the latest bootstrap version.`);
     }
 
     const outdir = await fs.mkdtemp(path.join(os.tmpdir(), 'cdk-bootstrap'));
@@ -114,4 +114,4 @@ export function bootstrapVersionFromTemplate(template: any): number {
     }
   }
   return 0;
-}
+}