Make Renovate update Makefile versions as well

[mowies]

Right now, Renovate doesn't take the Makefiles into account during version upgrades (as can be seen in the Detected dependencies here).

It would be very cool to get Renovate to update the versions in there as well, same as also in the pipeline.
Especially the k8s version at https://github.com/keptn/lifecycle-toolkit/blob/main/scheduler/Makefile#L33 would make a lot of sense, since that one actually has a lot of impact in the software.

Renovate can work with custom regexes to update things as well: https://docs.renovatebot.com/modules/manager/regex/