Rename `DesignVersion` model to `Action`

What does this MR do?

This MR renames the DesignVersion model to Action. The existing DesignVersion model is the join model between Design and Version.

As we often use this join table directly in Design Management, this change makes this model easier to reason about when using and describes what the data represents.

The relationships become:

design.actions
version.actions
 
action.design  
action.version 

It also avoids an awkward similarity in relation naming now that Issue.has_many :design_versions, class_name: "...::Version" !15561 (comment 212866249).

It does not rename the underlying database table.

#13825 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers