[Deploy and manage] Add CORS to ECH and Serverless differences

[KOTungseth]

Type of issue

I can't find what I'm looking for

What documentation page is affected

https://www.elastic.co/docs/deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings

What happened?

We've received external feedback that users are unable to find the CORs settings for Elastic Cloud Serverless.

It would be helpful to add CORs as a known difference between Elastic Cloud Hosted and Elastic Cloud Serverless.

Additional info

No response

[shainaraskas]

fixed in #1628