Bump the minimum CMake version to 3.20.

This should hopefully prevent us from being burned by CMake
dropping old version compatibility for a long time.

CMakeLists.txt

@@ -1,5 +1,5 @@
-# This usage of CMake requires at least version 3.8
-cmake_minimum_required(VERSION 3.8 FATAL_ERROR)
+# This usage of CMake requires at least version 3.20
+cmake_minimum_required(VERSION 3.20 FATAL_ERROR)
 
 # Set the project as C++ primarily, but have C enabled for the checks required later
 project(Anope CXX)

Config

@@ -139,7 +139,7 @@ done
 cmake --version 2>&1 > /dev/null
 if [ $? -ne 0 ] ; then
 	clear
-	echo "Anope requires CMake 3.8 or newer, which can be downloaded at https://cmake.org/ or through your system's package manager."
+	echo "Anope requires CMake 3.20 or newer, which can be downloaded at https://cmake.org/ or through your system's package manager."
 	echo "If you have installed CMake already, ensure it is in your PATH environment variable."
 	exit 0
 fi

docs/INSTALL

@@ -27,7 +27,7 @@ Note: You should also read the README and FAQ files!
         cmake --version
 
     If it's installed, you will get a line that says something similar to
-    "cmake version 3.28.3". If the version is less than 3.8.0 or you get
+    "cmake version 3.28.3". If the version is less than 3.20.0 or you get
     an error saying the command was not found, you will not be able to use
     CMake unless you install it yourself into your home directory. CMake
     can be downloaded from:

docs/INSTALL.fr

@@ -27,7 +27,7 @@ Note : Vous devrez également lire les fichiers README et FAQ !
         cmake --version
 
     Si CMake est installé, vous aurez une ligne qui dit quelque chose comme
-    "cmake version 3.28.3". Si la version est inférieure à 3.8.0 ou si vous
+    "cmake version 3.28.3". Si la version est inférieure à 3.20.0 ou si vous
     obtenez une erreur disant que la commande n'a pas été trouvée, vous ne
     pourrez pas utiliser CMake à moins de l'installer vous-même dans votre
     répertoire home. CMake peut être téléchargé ici :