Adjust Versions menu label

What does this MR do and why?

Adds a "v" to the Versions menu label to make it clearer that these are GitLab versions.

See https://gitlab.com/gitlab-org/gitlab-docs/-/issues/1675/designs/Desktop--Rest--Icon.png#note_1690972823

Screenshots, screen recordings, or links to review app

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Verify you see a "v" on the Versions drop-down menu
3. Verify links on the menu are still correct

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.