Update all deployments to use Postgres 13

Created by: slimsag

sourcegraph/server currently uses Postgres 11.5, which is not available in Alpine 3.12. To workaround this we have compiled Postgres 11.5 for Alpine 3.12: https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/cmd/server/Dockerfile#L44-51

This will in the future cause ongoing maintenance pain and we should update sourcegraph/server to use Postgres 13. Someone will need to look at how to perform that migration (I believe we did this in the past for postgres 10 -> 11?)

Additionally, there are security and performance improvements in Postgres 13.

@sourcegraph/cloud is this something you would be interested in performing?