[MIG] hr_shift: migrate to 19.0

[djordje-m]

Migrates hr_shift from 18.0 to 19.0.

Changes made

• Replace fields.Serialized with built-in fields.Json (removes base_sparse_field dependency)
• Replace hr.employee.base inheritance with hr.employee (abstract model removed in v19)
• Replace string_to_datetime import with pytz.utc.localize() (resource.models.utils refactored in v19)
• Rewrite _attendance_intervals_batch to use Intervals composition operators instead of ._items mutation
• Replace _sql_constraints with models.Constraint
• Replace module-level _() with self.env._()
• Remove category_id and users fields from res.groups record (removed in v19)
• Remove owl="1" from QWeb templates
• Add @api.model_create_multi to ShiftPlanningShift.create
• Fix fields.datetime → datetime (removed from odoo.fields in v19)

Testing

• All existing unit tests pass on Odoo 19

[victoralmau]

There is already another migration PR: #38, and besides, it doesn't have a commit history.