CMake 4 has dropped compatibility with old versions < 3.5, and will
drop compatibility for < 3.10 in the future. Update the minimum required
CMake version accordingly.
Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>
Change to zstd compressed instead of gz-compressed tarballs. zstd
compresses 7-8 times faster and decompresses 2-3 times faster.
This change also make sharing tarballs between different branches
possible, because OpenWrt 25.12 uses zstd as default.
Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>