chore: integ test trying to downgrade bootstrap stack (aws#17298)

This is a follow-up to aws#17277: we switched to *always* bootstrapping
the environment using the default settings, to automatically upgrade
whenever an upgrade was available.

However, if we run the integ test using a v1 CLI, the default bootstrap
stack will be the legacy bootstrap, and we would actually be trying
to *downgrade* it. Instead, always use the modern bootstrap stack.

Since legacy apps can be deployed to the modern bootstrap stack, this
is not an issue, and if a test actually needs the legacy stack to test
something, it will explicitly try to create a fresh legacy bootstrap
stack.


----

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

packages/aws-cdk/test/integ/helpers/cdk.ts

@@ -541,11 +541,21 @@ let sanityChecked: boolean | undefined;
  * by hand so let's just mass-automate it.
  */
 async function ensureBootstrapped(fixture: TestFixture) {
-  // use whatever version of bootstrap is the default for this particular version of the CLI
+  // Always use the modern bootstrap stack, otherwise we may get the error
+  // "refusing to downgrade from version 7 to version 0" when bootstrapping with default
+  // settings using a v1 CLI.
+  //
+  // It doesn't matter for tests: when they want to test something about an actual legacy
+  // bootstrap stack, they'll create a bootstrap stack with a non-default name to test that exact property.
   const envSpecifier = `aws://${await fixture.aws.account()}/${fixture.aws.region}`;
   if (ALREADY_BOOTSTRAPPED_IN_THIS_RUN.has(envSpecifier)) { return; }
 
-  await fixture.cdk(['bootstrap', envSpecifier]);
+  await fixture.cdk(['bootstrap', envSpecifier], {
+    modEnv: {
+      // Even for v1, use new bootstrap
+      CDK_NEW_BOOTSTRAP: '1',
+    },
+  });
   ALREADY_BOOTSTRAPPED_IN_THIS_RUN.add(envSpecifier);
 }