Renovate: disable lockFileMaintenance
Summary
This was added in !2064 (merged) as an attempt to make Renovate update yarn.lock, but the problem was actually with the GitLab Runner configuration and not the configuration of Renovate. The change in !2064 (merged) makes Renovate open an MR like !2091 (closed) to blast away and rebuild the whole lockfile, which we probably don't ever want.