Remove Ubuntu 20.04 from build instructions

doc/build_instructions/ubuntu.md

@@ -1,20 +1,8 @@
-# Prerequisite steps for Ubuntu users
+# Prerequisite steps for Ubuntu users (Ubuntu >= 21.04)
 
-You need at least Ubuntu 20.04 for `gcc >= 10` and Python 3.9.
-
-## Ubuntu >=21.04
+Run the following commands: 
 
  - `sudo apt-get update`
  - `sudo apt-get install g++-10 cmake cython3 libeigen3-dev libepoxy-dev libfontconfig1-dev libfreetype6-dev libharfbuzz-dev libogg-dev libopus-dev libopusfile-dev libpng-dev libsdl2-dev libsdl2-image-dev python3-dev python3-jinja2 python3-numpy python3-lz4 python3-pil python3-pip python3-pygments python3-toml qml-module-qtquick-controls qtdeclarative5-dev`
 
 You will also need [nyan](https://github.com/SFTtech/nyan/blob/master/doc/building.md) and its dependencies.
-
-
-## Ubuntu 20.04 LTS
-
-On Ubuntu 20.04 you need to install Python 3.9 manually as the default version provided is too old.
-
- - `sudo apt-get update`
- - `sudo apt-get install g++-10 cmake cython3 libeigen3-dev libepoxy-dev libfontconfig1-dev libfreetype6-dev libharfbuzz-dev libogg-dev libopus-dev libopusfile-dev libpng-dev libsdl2-dev libsdl2-image-dev python3.9 python3.9-dev python3.9-jinja2 python3.9-numpy python3.9-lz4 python3.9-pil python3.9-pip python3.9-pygments python3.9-toml qml-module-qtquick-controls qtdeclarative5-dev`
-
-You will also need [nyan](https://github.com/SFTtech/nyan/blob/master/doc/building.md) and its dependencies.