d/c-u-t.py: Ignore potentially non ESR versions

It happens that upstream is creating non beta or candidates versions
within the data for ESR and planned ESR versions.
We need to ignore such versions in the logic while looking for the
current most versions we are intressted in. Otherwise the script will
select such versions we don't want to see in the output.

There will be more versions to get ignored in the future, adding them to
the lists RELEASE_STRING_EXCLUSION_LIST and
PLANNED_STRING_EXCLUSION_LIST should be easy and prevent further code
modifications.