Use site_version to popupate the docsearch meta tag

From Sarah's comment:

The docsearch_version function that populates the docsearch:version meta tag did change not too long ago, in an attempt to fix the archives banner: !3448 (merged)

The change there was to reference docs.gitlab.com/versions.json instead of the site-local one, which would be outdated, which determining the site version, archive status, etc.

Seems like this could be a process flaw with our current code. Versions.json would've needed to be updated first so that the version you're cutting understands that it's no longer next at build time. Shoot.

To fix: The gitlab-docs-version metatag is correct for all these versions. We can probably utilize that logic for the docsearch tag.

Closes #1519 (closed)