[Documentation] Clarify "Update Fabric Eventhouse" step applicability in upgrade guide

[Copilot]

The upgrade guide's Step 2 previously stated that "The Microsoft Fabric eventhouse database schema must be manually updated with each release" without clarifying when this step applies, causing confusion for users who aren't using Microsoft Fabric as their primary data store.

Changes Made

Added conditional language to Step 2 that clearly indicates this step only applies when using Microsoft Fabric as a primary data store:

This step only applies if you are using Microsoft Fabric as a primary data store. Skip this step if any of the following apply:

- You are using Azure Storage as your data store  
- You are using Azure Data Explorer as your data store

Impact

This change resolves user confusion by:

• Clearly specifying when the Fabric eventhouse update is required
• Providing explicit skip conditions for other data store configurations
• Following the same conditional pattern used by other steps in the upgrade guide
• Maintaining all existing instructional content while adding necessary clarity

The change is minimal (5 lines added) and follows existing documentation patterns used in Steps 1, 4, and 5 of the same guide.

Fixes #1669.

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