helm_builder: remove whitespace from detected operator version

The lines returned by readlines contain the trailing newline (\n) character. If that is not removed, it ends up in the chart's version number, which helm does not like:

$ make k8s_helm_install_crds
[…]
Release "yaook-crds" does not exist. Installing it now.
Error: validation: chart.metadata.version "1.0.0\n" is invalid

Since whitespace is never part of a valid version number, we strip it away using .strip(). That makes it easier for humans to edit the version number file.