Remove mentions of 3.8 (pybamm-team#4291)

docs/source/user_guide/installation/gnu-linux-mac.rst

@@ -6,7 +6,7 @@ GNU/Linux & macOS
 Prerequisites
 -------------
 
-To use PyBaMM, you must have Python 3.8, 3.9, 3.10, 3.11, or 3.12 installed.
+To use PyBaMM, you must have Python 3.9, 3.10, 3.11, or 3.12 installed.
 
 .. tab:: Debian-based distributions (Debian, Ubuntu)
 
@@ -43,7 +43,7 @@ User install
 
 We recommend to install PyBaMM within a virtual environment, in order
 not to alter any distribution Python files.
-First, make sure you are using Python 3.8, 3.9, 3.10, 3.11, or 3.12.
+First, make sure you are using Python 3.9, 3.10, 3.11, or 3.12.
 To create a virtual environment ``env`` within your current directory type:
 
 .. code:: bash