[Reporting] Serverless reporting_user config false by default (#162835)

## Summary

This setting is deprecated and used when security features were less
developed. Moving forward in serverless, it would be beneficial to have
the `xpack.reporting.roles.allow: []` by default since backwards
compatibility shouldn't be as pressing.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Tim Sullivan <tsullivan@users.noreply.github.com>

x-pack/plugins/reporting/server/config/schema.test.ts

@@ -112,4 +112,14 @@ describe('Reporting Config Schema', () => {
       ConfigSchema.validate({ export_types: { csv: { enabled: true } } }, { dev: true })
     ).not.toThrow();
   });
+
+  describe('roles', () => {
+    it('should have roles enabled set to false for serverless by default', () => {
+      expect(ConfigSchema.validate({}, { serverless: true }).roles.enabled).toBe(false);
+    });
+
+    it('should have roles enabled set to true for non-serverless by default', () => {
+      expect(ConfigSchema.validate({}).roles.enabled).toBe(true);
+    });
+  });
 });

x-pack/plugins/reporting/server/config/schema.ts

@@ -84,8 +84,18 @@ const EncryptionKeySchema = schema.conditional(
 );
 
 const RolesSchema = schema.object({
-  enabled: schema.boolean({ defaultValue: true }), // true: use ES API for access control (deprecated in 7.x). false: use Kibana API for application features (8.0)
-  allow: schema.arrayOf(schema.string(), { defaultValue: ['reporting_user'] }),
+  enabled: schema.conditional(
+    schema.contextRef('serverless'),
+    true,
+    schema.boolean({ defaultValue: false }),
+    schema.boolean({ defaultValue: true })
+  ), // true: use ES API for access control (deprecated in 7.x). false: use Kibana API for application features (8.0)
+  allow: schema.conditional(
+    schema.contextRef('serverless'),
+    true,
+    schema.arrayOf(schema.string(), { defaultValue: [] }),
+    schema.arrayOf(schema.string(), { defaultValue: ['reporting_user'] })
+  ),
 });
 
 // Browser side polling: job completion notifier, management table auto-refresh