Update postgres references
Created by: david-sandy
Notes
All references to postgres-12.6-alpine have been replaced with postgres-12.7-alpine.
- docker-images/postgres-12.7-alpine > Dockerfile: The sha hash was updated.
- doc > admin > postgres.md: Line 70 is more concerning. sourcegraph/postgres-12.6:21-03-26_5d7084279 was replaced with sourcegraph/postgres-12.7. I assume the 12.6 tag was important. Please double check.
Test plan
More testing is needed, but, other than some local smoke tests, I am not certain what else to test.