Remove support for kernels older than 4.18

[Pinglinux]

We decided to remove support for kernels older than 4.18 since

RHEL 8 runs on kernel 4.18 Ubuntu 20.04, the oldest Long Term Support version, runs on kernel 5.4 Debian 11, will be supported until June 30, 2026, is running kernel 5.10

There is only one "Error", which was led by a warning irrelevant to the change:

WARNING: added, moved or deleted file(s), does MAINTAINERS need updating? #16: {4.5 => 4.18}/Makefile.in | 0 Error: WARNING: added, moved or deleted file(s), does MAINTAINERS need updating?

total: 0 errors, 1 warnings, 0 lines checked

Failure