Make v3.x.x the minimum required Mapnik version (#365) #399

Job	Run time
freebsd13 (CMake) (LLVM)	5m 41s
centos:7 (CMake) (GNU)	3m 57s
freebsd14 (CMake) (LLVM)	5m 18s
macos-12 (CMake) (LLVM)	7m 56s
debian:12 (Latest Mapnik) (CMake) (GNU)	2m 7s
debian:11 (CMake) (GNU)	2m 4s
macos-13 (Autotools) (LLVM)	11m 51s
ubuntu:22.04 (Latest Mapnik) (CMake) (GNU)	1m 43s
macos-13 (CMake) (LLVM)	8m 0s
debian:12 (CMake) (GNU)	1m 44s
debian:testing (CMake) (GNU)	1m 48s
fedora:38 (CMake) (GNU)	2m 14s
fedora:39 (CMake) (GNU)	2m 9s
fedora:rawhide (CMake) (GNU)	2m 22s
opensuse/leap:15 (CMake) (GNU)	4m 45s
ubuntu:20.04 (CMake) (GNU)	1m 40s
ubuntu:rolling (CMake) (GNU)	1m 33s
ubuntu:22.04 (Autotools) (GNU)	1m 41s
ubuntu:22.04 (Autotools) (LLVM)	1m 53s
ubuntu:22.04 (CMake) (GNU)	1m 36s
ubuntu:22.04 (CMake) (LLVM)	1m 50s
	1h 13m 52s

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