mkinitcpio: do not use zstd compression for kernels < 5.9
Support for zstd-compressed initramfs was added in Linux 5.9. Since LTS 5.4 is still supported by upstream until 2025-12 and SLTS 4.19 until 2029-01, add a fallback to gzip for Linux < 5.9 to avoid creating unbootable initramfs images.
Edited by nl6720