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

[jigpu]

Looking at the source of the checkpatch warning, it seems like it is not relevant to the input-wacom project. The warning is designed to ensure that if someone adds or removes files upstream, that they also make sure to update the MAINTAINERS file. We don't care about or even have a MAINTAINERS file, so this warning is meaningless for us.

The checkpatch script allows ignoring specific warnings. If we were running the script directly, adding --ignore FILE_PATH_CHANGES as an argument should fix this issue. Since we're running the script as an action though, it looks like we'll need to add a step to write a .checkpatch.conf file as described in the action documentation. We can make that a separate pull request though.

[jigpu]

Please squash the first and second commits together. This will ensure git bisect will work properly in the middle of this request.

[jigpu]

Thanks Ping, looks good to me.