update(build): relax dkms dependencies to suggestions in DEB and RPM packages

[jthiltges]

Using Falco with the modern_ebpf driver does not require kernel build dependencies. This PR updates the cmake configuration to make the dkms and kernel-devel package requirements into suggestions.

What type of PR is this?

/kind cleanup

Any specific area of the project related to this PR?

/area build

What this PR does / why we need it:

This makes the kernel-devel and dkms dependencies optional when installing the falco package, significantly reducing the number of packages installed when running in modern_ebpf mode.

On Alma 8, installing Falco 0.39.2 from RPM requires dkms and kernel-devel, which adds many dependencies for a total of 76 packages on my minimal test VM. After converting the dkms and kernel-devel requirements to suggestions with rpmrebuild, no additional dependencies beyond falco itself were required, and Falco appears to run fine with modern_ebpf. This PR should have the same effect on packages generated by cmake.

Does this PR introduce a user-facing change?:

YES

update(build): DEB and RPM package requirements for dkms and kernel-devel are now suggestions

[poiana]

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[poiana]

Welcome @jthiltges! It looks like this is your first PR to falcosecurity/falco 🎉

[FedeDP]

Hi! Thanks for this PR!
Indeed, we would love to improve Falco packages by splitting them for-driver, so that one can install falco-kmod that would install a Falco configured to run with the kernel module driver and with all kmod deps, a falco-ebpf and normal falco that would just drop all unneeded deps since it runs with modern ebpf driver.

This is something on our mind since ~1yr, but we couldn't find the time to work on it.
IMHO for now, this can be a good solution since the default Falco use case does not need any additional dep.

/cc @leogr @LucaGuerra