GHA: Update macOS runners

GHA: Update macOS runners

Remove macos-12, not supported anymore.
Add macos-15, runner is available in Beta status.

Change-Id: Ibaa591904095a824df2c0c5aaff73687e0af24fe
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Acked-by: Yuriy Darnobyt <yura.uddr@gmail.com>
Message-Id: <20241015205627.25644-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29551.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 6f74d395383eeca74d6cf6f87ee1387223b2bb1c)