Add Linux, Windows, DEB, and RPM standalone agent upgrade commands (#4187)

## Summary

The standalone agent upgrade documentation only showed macOS commands.
Added tabbed interface with simplified upgrade commands for all
supported platforms:

- **macOS**: `sudo elastic-agent upgrade 9.1.1`
- **Linux**: `sudo elastic-agent upgrade 9.1.1`
- **Windows**: `.\elastic-agent.exe upgrade 9.1.1` (as Administrator)
- **DEB**: `sudo elastic-agent upgrade 9.1.1`
- **RPM**: `sudo elastic-agent upgrade 9.1.1`

The instructions leverage the fact that step 2 already directs users to
navigate to the installation directory, so the code blocks only show the
upgrade command itself. For DEB/RPM installations, the elastic-agent
binary is already in PATH via systemd install.

Uses same tab-set syntax as `install-standalone-elastic-agent.md`.

## Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this
contribution?
- [x] Yes  
- [ ] No  

Tool(s) and model(s) used: GitHub Copilot (Claude)

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>[Fleet]: Standalone Agent Upgrade Commands not displayed
for Non-macOS</issue_title>
> <issue_description>**Description:**
> Upgrade standalone agents section only displays macOS upgrade
commands. Upgrade commands for other operating systems are missing.
> 
> **Link:**
> https://www.elastic.co/docs/reference/fleet/upgrade-standalone
> 
> 
> **Screen-shot:**
> 
> <img width="1601" height="740" alt="Image"
src="https://github.com/user-attachments/assets/f324c36a-2986-43ea-8822-9a654789e754"
/>
> 
> **Whats working fine** 
> - Installation commands are available for all operating systems.
> 
> **Expected Result**
> Upgrade commands for standalone agents should be available for all
supported operating systems/artifacts. </issue_description>
> 
> <agent_instructions>Here's the file:
https://github.com/elastic/docs-content/edit/main/reference/fleet/upgrade-standalone.md
> 
> Add Linux, Windows, DEB, and RPM instructions. An example of the
syntax is available here:
https://www.elastic.co/docs/reference/fleet/install-standalone-elastic-agent
(https://github.com/elastic/docs-content/edit/main/reference/fleet/install-standalone-elastic-agent.md).</agent_instructions>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes elastic/ingest-docs#1876

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: bmorelli25 <[email protected]>
Co-authored-by: Brandon Morelli <[email protected]>

Author: Copilot

reference/fleet/upgrade-standalone.md

@@ -13,13 +13,54 @@ To upgrade a standalone agent running on an edge node:
 1. Make sure the `elastic-agent` service is running.
 2. From the directory where {{agent}} is installed, run the `upgrade` command to upgrade to a new version. Not sure where the agent is installed? Refer to [Installation layout](/reference/fleet/installation-layout.md).
 
-    For example, on macOS, to upgrade the agent from version 8.8.0 to 8.8.1, you would run:
+    For example, to upgrade the agent from version 9.1.0 to 9.1.1, you would run:
+
+    :::::{tab-set}
+
+    ::::{tab-item} macOS
+
+    ```shell
+    sudo elastic-agent upgrade 9.1.1
+    ```
+
+    ::::
+
+    ::::{tab-item} Linux
+
+    ```shell
+    sudo elastic-agent upgrade 9.1.1
+    ```
+
+    ::::
+
+    ::::{tab-item} Windows
+
+    As an Administrator, run:
 
     ```shell
-    cd /Library/Elastic/Agent/
-    sudo elastic-agent upgrade 8.8.1
+    .\elastic-agent.exe upgrade 9.1.1
     ```
 
+    ::::
+
+    ::::{tab-item} DEB
+
+    ```shell
+    sudo elastic-agent upgrade 9.1.1
+    ```
+
+    ::::
+
+    ::::{tab-item} RPM
+
+    ```shell
+    sudo elastic-agent upgrade 9.1.1
+    ```
+
+    ::::
+
+    :::::
+
 
 This command upgrades the binary. Your agent policy should continue to work, but you might need to upgrade it to use new features and capabilities.
 
@@ -53,7 +94,7 @@ In the event of a private GPG key rotation, you can use the following options wi
     Example:
 
     ```yaml
-    ./elastic-agent upgrade 8.8.0 --skip-verify
+    ./elastic-agent upgrade 9.1.0 --skip-verify
     ```
 
 
@@ -63,7 +104,7 @@ In the event of a private GPG key rotation, you can use the following options wi
     Example:
 
     ```yaml
-    ./elastic-agent upgrade 8.8.0 --pgp-path /home/elastic-agent/GPG-KEY-elasticsearch
+    ./elastic-agent upgrade 9.1.0 --pgp-path /home/elastic-agent/GPG-KEY-elasticsearch
     ```