zoekt: relax alert threshold for get_indexing_options from 10 to 20 min

Created by: ggilmore

gitserver isn't HA, so critical_zoekt-indexserver_get_index_opt ends up firing whenever gitserver is unavailable during a rollout.

@sourcegraph/repo-management team has HA gitserver on their roadmap, so we're relaxing the alerting threshold to reduce fatigue until that lands.

See https://sourcegraph.slack.com/archives/C023ELQLV7F/p1639650293294000 for more context.