README.cmake.md: Document minimum required CMake version for --preset

README.cmake.md: Document minimum required CMake version for --preset

CMakePreset.json is supported since 3.19, but we have a version
3 preset file, so need at least 3.21.

Github: OpenVPN/openvpn#489
Change-Id: I44c555f6ffa08f2aee739c7f687fa3b678c86231
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20240201123039.174176-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28160.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>