This is a follow-up fix to an issue introduced by the changes
in !47069

Without formatting the timestamp datatypes returned by the
database, the lookup of each day can fail if GitLab is
configured to run with a Rails time_zone set to anything
non-UTC.

This happens due to the use of Date/Time objects for hash
lookups, which are influenced by the global Rails
configuration and produce a timezoned object that does not
equate with the same time in UTC.

Tests added here fail without the fix, displaying an all zero
data for the chart.

Fixes issue #296451