Remove mention of "elastic" user in cloud setup docs

[jsvd]

The docs under "Configure the output for Elasticsearch Service on Elastic Cloud" mention the "elastic" user, which we want to avoid so that the usage of the "elastic" super user account is discouraged. The link is:

https://github.com/elastic/beats/blame/main/libbeat/docs/output-cloud.asciidoc#L26

I was going to open a PR to change that "elastic" string, but I'm not sure if there's another user you want to suggest that could be referenced in other sections of the documentation.

/cc @nchaulet

[elasticmachine]

Pinging @elastic/obs-docs (Team:Docs)

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!

[dedemorton]

There are other places where we show the elastic user, so whoever works on this issue should do a global search.

We don't have a preferred replacement. I think something like username would be fine.

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!

[rehanpfmr]

@jsoriano @jsvd
Can this be assigned to me? I'm new to Opensource contribution and would like to take this up.

[rehanpfmr]

I'm working on this, I will open PR shortly @dedemorton

[rehanpfmr]

#40720