[FIX] project: Calculate the right task next occurrences dates

Steps to reproduce:

  - Install "Field Service" module
  - Go to settings and activate "Recurring Tasks"
  - Go to Field Service -> All tasks and create new one
  - Set for Planned Date : 10/01/2021 14:00 -> 10/02/2021 14:30
  - Check Recurrent and go to Recurrence tab
  - Select repeat every 1 weeks
  - Select Monday

Issue:

  Wrong date for next occurrences (first occurrence is 09/06 instead
  of 10/04).

Cause:

  Calculation based on today date.

Solution:

  Calculate next occurrences based on planned_start_date if set.

opw-2628777

closes odoo/odoo#75793

Related: odoo/enterprise#20529
Signed-off-by: LTU-Odoo <IT-Ideas@users.noreply.github.com>