Search backend: bump zoekt and use chunk matches

Created by: camdencheek

This updates the pinned version of Zoekt and updates our calls to the Zoekt search API to request ChunkMatches.

I decided to not put this behind a feature flag because it was a huge pain to thread that state through our search backend and across APIs, which would be necessary because searcher is also a client of Zoekt. Instead, there are two booleans that are set by this PR that can safely be switched to false in case we need to disable this behavior. This should be unnecessary (I've manually tested this pretty extensively in addition to updating tests in Zoekt), but I'm documenting it here just in case.

Test plan

Much manual testing as part of https://github.com/sourcegraph/zoekt/pull/367. Updated unit tests to work with chunk matches instead.