Clarify how ingest components are upgraded on Serverless

**Context**: [[REQUEST]: Serverless clarification for how agents get updated](https://github.com/elastic/ingest-docs/issues/1792)
**TL;DR**: Elastic Agents used with a Serverless project are not upgraded automatically. The user must take care of the upgrade of the Elastic Agents, and they can do it at their own convenience.

I've identified a couple of `deploy-manage` docs that could get a clarification based on that information:

- [Upgrade your deployment or cluster](https://www.elastic.co/docs/deploy-manage/upgrade/deployment-or-cluster): This includes a short sentence about Serverless being automatically upgraded, but it could probably use a note that (self-hosted) Elastic ingest components must be upgraded by the user.
- [Upgrade your ingest components](https://www.elastic.co/docs/deploy-manage/upgrade/ingest-components): Are Elastic Agent, Logstash and Beats self-hosted in all use cases? If we can safely say that 99% of the time they are, would it make sense to add Serverless to the `applies_to` in this doc, and try to figure out a different wording? Right now this doc is part of a user workflow for upgrading a deployment/cluster (other upgrade docs mention this one as their last step, e.g., [Upgrade on ECH](https://www.elastic.co/docs/deploy-manage/upgrade/deployment-or-cluster/upgrade-on-ech), [Upgrade on ECK](https://www.elastic.co/docs/deploy-manage/upgrade/deployment-or-cluster/upgrade-on-eck)), but if this doc applies to Serverless too, we need to figure out how to adjust this doc to fit the Serverless use case.
- [Answers to common serverless questions](https://www.elastic.co/docs/deploy-manage/deploy/elastic-cloud/serverless#general-what-is-serverless-elastic-answers-to-common-serverless-questions): We could add a Q&A here for this particular issue.
- [Architectural differences](https://www.elastic.co/docs/deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings#architectural-differences) and [Compare Elastic Cloud Hosted and Serverless](https://www.elastic.co/docs/deploy-manage/deploy/elastic-cloud#general-what-is-serverless-elastic-differences-between-serverless-projects-and-hosted-deployments-on-ecloud): For these two I am not entirely sure as they compare ECH and Serverless, and ingest components are separate from those, but I wonder if it makes sense to add a note somewhere that self-hosted Elastic ingest components are managed by the user in Serverless projects?

## Action
- look into proposed suggestions
- ensure we don't make it more complicated