Clarify that unprivileged mode is not supported for RPM and DEB packages

[vishaangelova]

Summary

This PR clarifies that unprivileged mode is not supported for RPM and DEB packages.

Resolves #4192

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Cursor 2.2.36 with claude-4.5-sonnet

[github-actions]

Vale Linting Results

Summary: 5 suggestions found

💡 Suggestions (5)

File	Line	Rule	Message
reference/fleet/elastic-agent-unprivileged.md	20	Elastic.FutureTense	'will perform' might be in future tense. Write in the present tense to describe the state of the product as it is now.
reference/fleet/elastic-agent-unprivileged.md	179	Elastic.Acronyms	'RPM' has no definition.
reference/fleet/elastic-agent-unprivileged.md	179	Elastic.Acronyms	'DEB' has no definition.
reference/fleet/elastic-agent-unprivileged.md	253	Elastic.Acronyms	'RPM' has no definition.
reference/fleet/elastic-agent-unprivileged.md	253	Elastic.Acronyms	'DEB' has no definition.

[github-actions]

🔍 Preview links for changed docs

• reference/fleet/elastic-agent-unprivileged.md