Update build instructions to include .NET 9 (#4109)

BUILDING.md

@@ -16,28 +16,30 @@ build.cmd -help
 
 ## Table of contents
 
-- [Prerequisites](#prerequisites)
-- [Build roadmap](#build-roadmap)
-- [Building IceRpc](#building-icerpc)
-  - [Command line](#command-line)
-  - [Visual Studio Code](#visual-studio-code)
-- [Running the tests](#running-the-tests)
-  - [C# tests](#c-tests)
-  - [Rust tests](#rust-tests)
-- [Creating and publishing NuGet packages](#creating-and-publishing-nuget-packages)
-  - [Slice tools](#slice-tools)
-- [Generating the API reference](#generating-the-api-reference)
-- [Generating the code coverage reports](#generating-the-code-coverage-reports)
-- [Shutting down background MSBuild servers](#shutting-down-background-msbuild-servers)
-- [Updating Slice files](#updating-slice-files)
+- [Building from source](#building-from-source)
+  - [Table of contents](#table-of-contents)
+  - [Prerequisites](#prerequisites)
+  - [Build roadmap](#build-roadmap)
+  - [Building IceRpc](#building-icerpc)
+    - [Command line](#command-line)
+    - [Visual Studio Code](#visual-studio-code)
+  - [Running the tests](#running-the-tests)
+    - [C# tests](#c-tests)
+    - [Rust tests](#rust-tests)
+  - [Creating and publishing NuGet packages](#creating-and-publishing-nuget-packages)
+    - [Slice tools](#slice-tools)
+  - [Generating the API reference](#generating-the-api-reference)
+  - [Generating the code coverage reports](#generating-the-code-coverage-reports)
+  - [Shutting down background MSBuild servers](#shutting-down-background-msbuild-servers)
+  - [Updating Slice files](#updating-slice-files)
 
 ## Prerequisites
 
 1. Rust\
 Install Rust using [rustup](https://rustup.rs/).
 
-2. .NET SDK 8.0\
-Download the .NET SDK from [dotnet.microsoft.com](https://dotnet.microsoft.com/en-us/download/dotnet).
+2. .NET SDK 8.0 and 9.0 (both are required)\
+Download the .NET SDKs from [dotnet.microsoft.com](https://dotnet.microsoft.com/en-us/download/dotnet).
 
 3. docfx (optional)\
 The IceRRC API reference is generated by [docfx](https://www.nuget.org/packages/docfx). You can install docfx as