Swap out ref_switcher on blob viewer

What does this MR do and why?

Swap out the branch selector (ref-selector) from the haml one to the Pajamas Vue component version.

Screenshots or screen recordings

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

How to set up and validate locally

• In a project go to "Repository" in the left nav
• Select a single file to view
• Verify the branch selector works

MR 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 MR.

Related to #369960 (closed)