Finalize workspaces config version3 migration & cleanup old migration specs

Issue: Backend: Rename config_version field to desired... (#479693 - closed)

What does this MR do and why?

Why this is needed This MR finalizes the UpdateWorkspacesConfigVersion3 background migration and also performs some cleanup and removal of an old spec which already had its migration finalized here MR.

Finalization of old background migrations is a pre-requisite for safe removal of old migration code & its specs to ensure that said migration has completed.

Related to:

• Add column workspaces_agent_config_version and ... (!165912 - merged)
• Rename config_version field to desired_config_g... (!165876 - merged)

Where specs for these old background migrations fail and cause issues

• ee/spec/lib/gitlab/background_migration/update_workspaces_config_version_spec.rb
• ee/spec/lib/gitlab/background_migration/update_workspaces_config_version3_spec.rb

What it does Finalize UpdateWorkspacesConfigVersion3 migration

• Adds a post migration to ensure background migration is finished
• Updates finalized_by in background migration doc
• Deletes spec for the already finalized migration UpdateWorkspacesConfigVersion
• Pins the spec for UpdateWorkspacesConfigVersion3 to the finalized schema version in this MR

Changelog: changed

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