Backend: Rename config_version field to desired_config_generator_version on workspaces table

MRs:

• Finalize workspaces config version3 migration &... (!167094 - merged)
• Rename config_version field to desired_config_g... (!165876 - merged)
• Add migration step to safeguard rename config_v... (!168670 - merged)

Description

There is an existing column config_version, it is confusing as we are having a new column workspace_agent_config_version.

We would like to rename the column to desired_config_generator_version.

This issue is to add a new column, and also update application code to reference on this new column.

Also check all data for existing values, and new column should have default of the same version value.

GitLab migration docs:

• https://docs.gitlab.com/ee/development/migration_style_guide.html#how-long-a-migration-should-take
• https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#renaming-columns

Acceptance Criteria

• check prod data on all workspaces
• migration to create a new desired_config_generator_version column with default: 3
• migration to copy data from config_version to desired_config_generator_version
• update reference to this existing column in application code