Remove compatibility with old cmake (<3.5). #637

Job	Run time
mmg-debug / ci (ubuntu-20.04, on, off, on, off, int32_t)	1h 16m 51s
mmg-debug / ci (ubuntu-20.04, on, off, off, off, int32_t)	1h 35m 7s
mmg-debug / ci (ubuntu-20.04, off, off, on, off, int32_t)	1h 3m 9s
mmg-debug / ci (ubuntu-20.04, off, off, off, off, int32_t)	1h 12m 50s
mmg-debug / ci (macos-12, on, off, on, off, int32_t)	47m 48s
mmg-debug / ci (macos-12, on, off, off, off, int32_t)	1h 2m 23s
mmg-debug / ci (macos-12, off, off, on, off, int32_t)	34m 54s
mmg-debug / ci (macos-12, off, off, off, off, int32_t)	43m 50s
mmg-debug / ci (ubuntu-20.04, off, off, off, on, int32_t)	4m 36s
mmg-debug / ci (macos-12, off, off, off, on, int32_t)	9m 46s
mmg-debug / ci (ubuntu-20.04, off, on, on, off, int32_t)	1h 3m 15s
mmg-debug / ci (macos-12, off, on, on, off, int32_t)	37m 10s
mmg-debug / ci (windows-2022, off, off, off, off, int32_t)	56m 24s
mmg-debug / ci (windows-2022, on, off, off, off, int32_t)	53m 25s
mmg-debug / ci (windows-2022, off, on, off, off, int32_t)	54m 46s
mmg-debug / ci (ubuntu-20.04, on, on, on, on, int64_t)	4m 36s
mmg-debug / ci (ubuntu-20.04, off, off, on, on, int64_t)	1h 10m 21s
mmg-debug / ci (ubuntu-20.04, off, off, off, off, int64_t)	2m 18s
mmg-debug / ci (macos-12, on, on, on, on, int64_t)	13m 47s
mmg-debug / ci (macos-12, off, off, off, off, int64_t)	5m 26s
mmg-debug / ci (windows-2022, on, on, off, off, int64_t)	4m 48s
mmg-debug / ci (windows-2022, off, off, off, off, int64_t)	4m 47s
mmg-debug / upload_coverage	2m 38s
	14h 44m 55s

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