This is a follow-up fix to an issue introduced by the changes
in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/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 https://gitlab.com/gitlab-org/gitlab/-/issues/296451