Implement backfill migration for `uuid` and `version` fields in `vscode_settings` table

Problem

The uuid and version fields in the vscode_settings table require non-null values for the correct functioning of the Settings Sync feature. In !133668 (merged), we introduced these fields and the backend logic sets default values. However, the database doesn't enforce the not null constraint.

Solution

Following the not null constraint database guidelines, we should implement a backfill migration to ensure that existing records have values set for the version and uuid fields. Afterward, we will enforce the not null constraint in these fields.