fix: Removing references to OpenSearch proxy

[grlimacan]

Description

In https://github.com/camunda/camunda-optimize/issues/10058 we have decided to de-scope the implementation for proxy for OpenSearch, so we are updating the documentation to reflect that

Same applies to the response buffer limit as described in camunda/camunda#24083

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.

[github-actions]

The preview environment relating to the commit af3869e has successfully been deployed. You can access it at https://preview.docs.camunda.cloud/pr-4549/index.html

[conceptualshark]

This looks good to me from my understanding of the linked issues, but I'm not certain if an engineering review would be prudent. Just to be certain, it seems like the ability to use the proxy was not ever implemented, only documented, and so there is no need to announce the removal?