[Elastic Agent] Clarify that Elastic Agent unprivileged mode is not supported for RPM and DEB packages

[swiatekm]

Elastic Agent can run in unprivileged mode, but this doesn't work for agents installed using DEB and RPM packages, and never has. See elastic/elastic-agent#9918 for reference. Until this is properly fixed, we should make it clear to users that this doesn't work, and there aren't any workarounds.

[swiatekm]

@nimarezainia @cmacknz FYI, we got an SDH about this recently and the lack of an explicit statement in the documentation made resolving it more complicated than it needed to be.