Test against PG9 by default, PG11 in master, and PG10 nightly

Current versions testing

Where?	PG version
MRs	9.6
master (non-scheduled pipelines)	9.6 / 11
2-hourly scheduled pipelines	9.6 / 11
Nightly scheduled pipeline	9.6 / 10

Long-term plan

We follow the PostgreSQL versions shipped with Omnibus GitLab:

PG version	12.10 (April 2020)	13.0 (May 2020)	13.1 (June 2020)	13.2 (July 2020)	13.3 (August 2020)	13.4, 13.5	13.6 (November 2020)	14.0 (May 2021?)
PG9.6	MRs/master/2-hour/nightly	-	-	-	-	-	-	-
PG10	nightly	-	-	-	-	-	-	-
PG11	master/2-hour	MRs/master/2-hour/nightly	MRs/master/2-hour/nightly	MRs/master/2-hour/nightly	MRs/master/2-hour/nightly	MRs/master/2-hour/nightly	nightly	-
PG12	-	-	-	-	master/2-hour	master/2-hour	MRs/master/2-hour/nightly	master/2-hour
PG13	-	-	-	-	-	-	-	MRs/master/2-hour/nightly

Closes #215053 (closed)