Update MCP Server project template install instructions

[jeffhandley]

With Introduce local vs. remote mcpserver template option (dotnet/extensions#7168), the mcpserver project template has been moved to a new package Microsoft.McpServer.ProjectTemplates.

This updates the only reference to the project template with the new installation instructions.

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Internal previews

📄 File	🔗 Preview link
docs/ai/quickstarts/build-mcp-server.md	Create a minimal MCP server using C# and publish to NuGet

[Copilot]

Pull request overview

This PR updates the MCP Server project template installation instructions to reference the new package name Microsoft.McpServer.ProjectTemplates instead of the deprecated Microsoft.Extensions.AI.Templates.

• Updates the template installation command to use the new package
• Updates the NOTE section to reference the correct package name requirement

[KalleOlaviNiemitalo]

The other note higher up on the page still references Microsoft.Extensions.AI.Templates, which seems out of place now that the package is not mentioned elsewhere on the page:

docs/docs/ai/quickstarts/build-mcp-server.md

Lines 14 to 15 in 25cf86b

	> [!NOTE]
	> The `Microsoft.Extensions.AI.Templates` experience is currently in preview. The template uses the [ModelContextProtocol](https://www.nuget.org/packages/ModelContextProtocol/) library and the [MCP registry `server.json` schema](https://github.com/modelcontextprotocol/registry/blob/main/docs/reference/server-json/generic-server-json.md), which are both in preview.