Fix documentation of supported PostgreSQL version (#11725)

Signed-off-by: Sebastian Hasler <sebastian.hasler@stuvus.uni-stuttgart.de>

changelog.d/11725.doc

+Document that now the minimum supported PostgreSQL version is 10.

docs/postgres.md

 # Using Postgres
 
-Synapse supports PostgreSQL versions 9.6 or later.
+Synapse supports PostgreSQL versions 10 or later.
 
 ## Install postgres client libraries