The auto column conversion should be limited to simple and intuitive use cases. It shouldn't trigger the slow ORM recomputation if the field is computed. We always expect an upgrade script to handle more complex use cases.

This commit introduces two changes:

### 1. Removal of `drop_not_null` during auto column conversion
Before https://github.com/odoo/odoo/commit/50767ef90eadeca2ed05b9400238af8bdbe77fb3 We dropped the not_null constraint because the original column would be renamed. After that commit, we actually don't need to drop the not_null constraint since the `convert_column` will neither convert a not-null value to `null` nor convert 'null' to a not-null value. Keeping the not_null constraint shouldn't block the column convert.

### 2. Removal of `column.clear()` 
When a computed/related Float field is changed from `digits=None` to `digits='xxx'`, the `column.clear()` will trigger ORM recomputation during upgrade which is useless since `double precision` to `numeric` is lossless. The recomputation in ORM is slow and should be avoided. If the rerounding is really needed, a sql script is required for upgrade or installation.

The `column.clear()` was originally introduced to avoid `Missing not-null constraint` warnings in specific scenarios:
Case 1 (Upgrade Warning): from saas-18.4 to 19.0
old database: Selection field `l10n_be.export.sdworx.leaves.wizard.reference_year` upgrade: pre-migrate `util.rename_model(cr, "l10n_be.export.sdworx.leaves.wizard", "l10n.be.hr.payroll.export.sdworx")` new database: Integer field `l10n.be.hr.payroll.export.sdworx.reference_year` The column value which was a required stringified integer is auto-converted to an integer.
Case 2 (Installation Warning):
In pos_urban_piper, the required field `pos.config.name` is overridden from `translate=False` to `translate=True`. The column value which was a required text is auto-converted to `'{"en_US": "text"}'::jsonb`

The not_null constraint was previously lost by the `sql.drop_not_null` in `update_db_column` and is not restored by `update_db_notnull` because of the inconsistency between the variable `column['is_nullable']` and the actual not_null constraint in the database.

Thanks to change 1, we will no longer lose the not_null constraint in `update_db_column`. The constraint can be kept even without `column.clear()`.

By removing the `column.clear()`, we also revert the meaning of the `column` variable, which is the column's configuration (dict) before `update_db` if it exists, or `None`

Description of the issue/feature this PR addresses:

Current behavior before PR:

Desired behavior after PR is merged:




---
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

Forward-Port-Of: odoo/odoo#236299