CI: remove 4.14 and 4.19 from CI

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Both kernels are now EOL'd by kernel.org. Cilium documents that it requires at least 5.4 (or a RHEL equivalent).

I propose that we remove 4.14 and 4.19 from CI but keep exported APIs as they are right now. This means that existing functionality probably won't break on older kernels. New functionality isn't require to cater to them anymore.

Please be aware that distribution kernels may have the same version number but are effectively distinct from upstream kernel.org. Best example is RHEL where a 4.18 kernel has most BPF features from mainline 5.4 backported. Another case is ubuntu kernels breaking BPF in various non-upstream ways.

So needing to support a distribution kernel is distinct from supporting mainline kernels, in my opinion.