[FIX] project: correct pager of project portal view

Current behaviour:
If a project has more than 80 tasks, there is pagination activated
in the project portal view. But when clicking on the second page, we
are greeted with a 404.

Expected behaviour:
You should be able to scroll through the pages of tasks related to
the shared project without hitting a 404.

Steps to reproduce:
- Install Project
- Create 100+ tasks in 1 project
- Copy the share link of that project.
- Log out, open the shared link.
- Go to page 2 of the tasks -> 404.

Reason for the problem:
Missing route `/my/project/<int:project_id>/page/<int:page>`.

Fix:
Add the missing route.
Essentially a backport of 336befc0

Affected versions:
- 15.0
- saas-15.2
- 16.0       <-- issue is different for this version and newers
- saas-16.1
- saas-16.2
- master

opw-3220659

closes odoo/odoo#119905

Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>