This PR adds a helpful tip to the upgrade documentation that informs users about the --force flag when installing .NET Aspire project templates.

Problem

When users already have the .NET Aspire workload installed and try to install project templates using dotnet new install Aspire.ProjectTemplates, they may encounter issues if existing templates need to be overwritten. The documentation didn't previously mention the --force flag option.

Solution

Added a TIP block immediately after the project templates installation code block in docs/get-started/upgrade-to-aspire-9.md that:

• Explains when to use the --force flag (when the workload is already installed)
• Provides a cross-reference to the workload removal section for users who prefer to uninstall instead

Changes Made

• Added a single TIP block with proper markdown formatting
• Included an internal link to the existing "Remove the .NET Aspire workload" section
• Followed existing documentation patterns and formatting conventions

The change is minimal and surgical - only 3 lines added without modifying any existing content. The tip provides valuable guidance for users who might encounter template installation conflicts during upgrades.

This pull request was created as a result of the following prompt from Copilot chat.

Add a tip about using the --force flag when installing .NET Aspire project templates if the workload is already installed, and refer to the workload removal section at the bottom of the doc for uninstall instructions.

Insert the following after the code block for installing project templates in docs/get-started/upgrade-to-aspire-9.md:

[!TIP]
If you already have the .NET Aspire workload installed, you need to pass the --force flag to overwrite the existing templates. For instructions on uninstalling the workload, see Remove the .NET Aspire workload (first-time upgrades from version 8 only).

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