Update BulkImports http client to use /metadata to fetch source version

What does this MR do and why?

• This MR updates BulkImports::Clients::HTTP to use /metadata endpoint when fetching source GitLab instance version when /version endpoint is not available
• /version endpoint is planned to be removed in 16.0 #366287 (closed)
• The client still uses /version endpoint in order to provide backwards compatibility, if we're migrating from older GitLab instance that does not have /metadata endpoint (it was introduced in 15.2)
• Later on, past 16.0, we can start using /metadata endpoint first and then fallback to /version, as it feels like there are more gitlab instances with /version endpoint vs /metadata endpoint

Mentions #366319 (closed)

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