Update Sourcegraph Docker images Docker tags to v3.1.0

[renovate]

This PR contains the following updates:

Package	Update	Change
sourcegraph/frontend	patch	3.1.0-rc.4 -> 3.1.0@467e9fb
sourcegraph/github-proxy	patch	3.1.0-rc.4 -> 3.1.0@087f1e8
sourcegraph/gitserver	patch	3.1.0-rc.4 -> 3.1.0@f3cd149
sourcegraph/management-console	patch	3.1.0-rc.4 -> 3.1.0@ae856a5
sourcegraph/query-runner	patch	3.1.0-rc.4 -> 3.1.0@607aa3a
sourcegraph/repo-updater	patch	3.1.0-rc.4 -> 3.1.0@c004cdd
sourcegraph/searcher	patch	3.1.0-rc.4 -> 3.1.0@c11577d
sourcegraph/symbols	patch	3.1.0-rc.4 -> 3.1.0@ab36535

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