docs: Replace old note about Postgres 10+ being required with link to our...

docs: Replace old note about Postgres 10+ being required with link to our deprecation policy (#14590)

changelog.d/14590.doc

+Remove old, incorrect minimum postgres version note and replace with a link to the [Dependency Deprecation Policy](https://matrix-org.github.io/synapse/v1.73/deprecation_policy.html).
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docs/postgres.md

 # Using Postgres
 
-Synapse supports PostgreSQL versions 10 or later.
+The minimum supported version of PostgreSQL is determined by the [Dependency
+Deprecation Policy](deprecation_policy.md).
 
 ## Install postgres client libraries