revert workspaces_agent_config_version validation

Issue - Backend Migration: New field for workspaces_age... (#479461 - closed)

What does this MR do and why?

We have an issue with existing workspace during intermediate state of the previous MR.

As a fix, we should temporary disable the validation until 17.6

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

• unrun migration first to mimic the behaviour, db/post_migrate/20240923015832_add_not_null_to_workspaces_agent_config_versions.rb
• create some workspaces followed by the local doc, https://gitlab.com/gitlab-org/workspaces/gitlab-workspaces-docs/-/blob/main/doc/local-development-environment-setup.md?ref_type=heads
• then rails c and foo = RemoteDevelopment::Workspace.last. foo.workspaces_agent_config_version = nil, foo.save!
• and see no issue raised