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.

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