gomod: update zoekt

Created by: keegancsmith

Quite a few changes since our last bump:

• https://github.com/sourcegraph/zoekt/commit/ae47cfc indexserver: ensure indexed for unchanged repositories
• https://github.com/sourcegraph/zoekt/commit/905148e indexserver: set and read Config Fingerprint
• https://github.com/sourcegraph/zoekt/commit/8cc3582 indexserver: Sourcegraph interface is just List
• https://github.com/sourcegraph/zoekt/commit/d89a313 indexserver: move CloneURL into IndexOptions
• https://github.com/sourcegraph/zoekt/commit/bfeddd7 indexserver: delete meta file after merging
• https://github.com/sourcegraph/zoekt/commit/cd787b0 update README to reflect we are the defacto source
• https://github.com/sourcegraph/zoekt/commit/f8682c1 shards: stream results per repo for compound shards
• https://github.com/sourcegraph/zoekt/commit/ba7cd37 indexserver: merge in order of priority
• https://github.com/sourcegraph/zoekt/commit/004c956 shards: send max pending priority while searching
• https://github.com/sourcegraph/zoekt/commit/df63d76 indexserver: don't delete shards if merge didn't run
• https://github.com/sourcegraph/zoekt/commit/4c69cfb indexserver: automate merging
• https://github.com/sourcegraph/zoekt/commit/9d85eab indexserver: update merge policy
• https://github.com/sourcegraph/zoekt/commit/0e81eb3 shards: set SearchResult.Progress.Priority
• https://github.com/sourcegraph/zoekt/commit/c79ecf9 shards: only observeMetrics when r.err == nil
• https://github.com/sourcegraph/zoekt/commit/f493172 shards: buffer search channel