Remove default value for actual_state_updated_at on workspaces table

MR: Remove default for actual_state_updated_at (!192860 - merged)

Description

Remove default value for actual_state_updated_at on workspaces table.

This is a follow-up for the discussion discussion on Add actual_state_updated_at column to workspace... (!184104 - merged) • Chad Woolley • 17.10:

> NOTE: Default value does not matter, as long as it is in the past and before any possible occurrence of Workspace reconcilation.
> 
> This only applies to existing workspace records, since we will explicitly set this field to `Time.current.utc` in the model in a future MR https://gitlab.com/gitlab-org/gitlab/-/merge_requests/184452+
> 
> We could remove the default in a future release migration if we wanted, just to be consistent with `desired_state_updated_at`, but it's not necessary.

This should be done in a release after %17.10 , so that we are sure that the previous migration has run and populated all the existing records with the default

Reference

• Startup scripts for Remote Development workspac... (&15602)