chore(docs): update flutter version (remove install using git instructions to simplify docs)

Author: julian-CStack

docs/building.md

@@ -77,16 +77,7 @@ pip3 install --upgrade meson==0.64.1 markdown==3.4.1 markupsafe==2.1.1 jinja2==3
 ```
 
 ### Flutter
-Install Flutter 3.35.7 by [following their guide](https://docs.flutter.dev/get-started/install/linux/desktop?tab=download#install-the-flutter-sdk).  You can also clone https://github.com/flutter/flutter, check out the `3.35.7` tag, and add its `flutter/bin` folder to your PATH as in
-```sh
-FLUTTER_DIR="$HOME/development/flutter"
-git clone https://github.com/flutter/flutter.git "$FLUTTER_DIR"
-cd "$FLUTTER_DIR"
-git checkout 3.35.7
-echo 'export PATH="$PATH:'"$FLUTTER_DIR"'/bin"' >> "$HOME/.profile"
-source "$HOME/.profile"
-flutter precache
-```
+Install Flutter 3.38.5 by [following their guide](https://docs.flutter.dev/install/manual).
 
 Run `flutter doctor` in a terminal to confirm its installation.
 
@@ -227,7 +218,7 @@ rustup target add aarch64-apple-ios aarch64-apple-darwin
 Optionally download [Android Studio](https://developer.android.com/studio) as an IDE and activate its Dart and Flutter plugins.  VS Code may work as an alternative, but this is not recommended.
 
 ### Flutter
-Install [Flutter](https://docs.flutter.dev/get-started/install) 3.29.2 on your Mac host by following [these instructions](https://docs.flutter.dev/get-started/install/macos).  Run `flutter doctor` in a terminal to confirm its installation.
+Install 3.38.5 on your Mac host by [following their guide](https://docs.flutter.dev/install/manual).  Run `flutter doctor` in a terminal to confirm its installation.
 
 ### Build plugins and configure
 #### Building plugins for iOS 
@@ -293,16 +284,7 @@ If the DLLs were built on the WSL filesystem instead of on Windows, copy the res
 Frostdart will be built by the Windows host later.
 
 ### Install Flutter on Windows host
-Install Flutter 3.35.7 on your Windows host (not in WSL2) by [following their guide](https://docs.flutter.dev/get-started/install/windows/desktop?tab=download#install-the-flutter-sdk) or by cloning https://github.com/flutter/flutter, checking out the `3.35.7` tag, and adding its `flutter/bin` folder to your PATH as in
-```bat
-@echo off
-set "FLUTTER_DIR=%USERPROFILE%\development\flutter"
-git clone https://github.com/flutter/flutter.git "%FLUTTER_DIR%"
-cd /d "%FLUTTER_DIR%"
-git checkout 3.35.7
-setx PATH "%PATH%;%FLUTTER_DIR%\bin"
-echo Flutter setup completed. Please restart your command prompt.
-```
+Install Flutter 3.38.5 on your Windows host (not in WSL2) by [following their guide](https://docs.flutter.dev/install/manual).
 
 Run `flutter doctor` in PowerShell to confirm its installation.