dist/redhat: add a proper changelog entry

[tchaikov]

it's distracting when trying to find build / test errors.

without this change, rpmbuild complains at see it like:

16:44:37  error: bad date in %changelog: initial release
16:44:37  warning: source_date_epoch_from_changelog set but %changelog
is missing

[tchaikov]

see https://jenkins.scylladb.com/job/scylla-master/job/next/5765/consoleFull for an instance of the error message.

[tchaikov]

@fruch hi Israel, could you help review this change?