Before this, the master version is the next major version without taking
intermediate saas versions into account.

Depending on the point of view, one or other choice can make more sense. Anyway,
master is always closer to next saas than next release.

Using next major release can be problematic from a technical reason, mainly for
upgrade scripts
A simple exemple is to write upgrade scripts for master. The script version
will be something like 13.3.1.2 since next saas is saas 13.3 and module version
1.2. The problem here is that installing a database in master will have module
versions setted to  14.0.1.2 since release.py indicates that version is 14.0,
making it impossible to test an incremental version script 13.3.1.3 in master
(will never be executed, 13.3.1.3 < 14.0.1.2)

Another exemple is when testing migrations on a not-rebased branch.
Hypotetically if all upgrade scripts are linked to a specific
version of a module (module version is incremented each time a
upgrade script is added), someone using an old version of the master
won't have the script 13.3.1.3 executed if current module version is 1.2.
(which is not true now since module version will be considered to be
14.0.1.2 (> 13.3.1.2).

closes odoo/odoo#47281

Signed-off-by: Olivier Dony (odo) <odo@openerp.com>