add indication that postgres upgrade might take a few minutes

cc @tsenart

Address Postgres test plan feedback (sourcegraph#2195)

* docs: Emphasize Postgres upgrade docs CTA

* Emojify Postgres docs

* cmd/server: Improve UX of error messages in Postgres upgrade

* Document downtime requirement

* postgres: Fix permissions of upgrade dir

* De-emojify

* More clear switch statement logic

* Much clearer upgrade instructions

* Update doc/admin/postgres.md

Co-Authored-By: tsenart <[email protected]>

* Update doc/admin/postgres.md