Fix: Update day grid to handle DLS

What does this MR do?

A tiny MVC for: https://gitlab.com/gitlab-org/gitlab/-/issues/323944 where we handle the generated gap for DLS by allowing a rotation assignee to "absorb" the extra/less space generated by a change in timezone via the offset. This works for both directions for the timezone offset(+/-).

We also update the shift pop over to use the locale offset rather than the selected timezone offset as when DLS changes, this becomes invalid. This lets us remove a selection of now-redundant code in the form of the selectedTimezone injection 😄

Screenshots (strongly suggested)

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Related to #323944