Update pre-migration.py

[Weltschmerz-dev]

First time contributor, hi 👋

Fixes runtime error due to invalid migration script.

Stacktrace:

File "/oca/server-auth/auth_session_timeout/migrations/17.0.1.0.0/pre-migration.py", line 6, in migrate
    env.cr.execute(
    ^^^^^^
  File "/odoo/odoo/odoo/sql_db.py", line 494, in __getattr__
    return getattr(self._obj, name)
           ^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'psycopg2.extensions.cursor' object has no attribute 'cr'

[amh-mw]

Code review only. Style coherent with core Odoo 17.0 pre-migrations.