Make the versions dropdown dynamic

What does this MR do and why

This is a rebuild of the Versions drop-down menu. The new component:

• Uses Vue and GitLab UI
• Pulls docs versions from a JSON file, allowing older versions to pick up changes dynamically when main is updated
• Uses different wording for menu items (see technical-writing#536 (comment 1017243538))
• Hides the menu for self-hosted docs sites, which only contain a single version (see #1101 (closed))

This will allow for a single MR (instead of ~5) to update the versions menu when deploying a new docs release.

Screenshots

Before	After

Testing

1. Load the review app
2. View the versions drop-down with a few different screen sizes and/or browsers

Note that links to old versions will not work in the review app, but you can look at the URL paths and verify those exist on the production site.

Closes #667 (closed), #1101 (closed)