[15.0][FIX] hr_timesheet_sheet_attendance: Fix timezone handling when linking attendances

[AaronHForgeFlow]

A fix was proposed in #845 but this one goes more straight to the point IMHO.

Root cause: The retroactive linking in HrTimesheetSheet.create() compared the UTC-stored check_in datetime against bare dates using midnight UTC as boundary. For UTC-negative timezones, any check-in on
date_end (e.g. 17:57 UTC) fails check_in <= date_end 00:00 UTC.

Steps to reproduce:

1. Create an employee with timezone set to a UTC-negative timezone (e.g. America/New_York)
2. Create an attendance for that employee with check_in on the last day of the intended sheet period (e.g. Sunday), at any normal working hour
3. Create a timesheet sheet covering that period after the attendance was created
4. Observe that the attendance is not linked to the sheet (sheet_id is empty)

[AaronHForgeFlow]

pre-commit will be fix after this being merged: #890