docs(cloudsql): complete POSTGRES version list (#3568) (#2108)

Co-authored-by: Bastien Dronneau <[email protected]>
Signed-off-by: Modular Magician <[email protected]>

Co-authored-by: Bastien Dronneau <[email protected]>

Author: modular-magician

.changelog/3568.txt

@@ -0,0 +1,3 @@
+```release-note:none
+
+```

website/docs/r/sql_database_instance.html.markdown

@@ -194,7 +194,7 @@ The following arguments are supported:
 
 * `database_version` - (Optional, Default: `MYSQL_5_6`) The MySQL, PostgreSQL or
 SQL Server (beta) version to use. Supported values include `MYSQL_5_6`,
-`MYSQL_5_7`, `POSTGRES_9_6`,`POSTGRES_11`, `SQLSERVER_2017_STANDARD`,
+`MYSQL_5_7`, `POSTGRES_9_6`,`POSTGRES_10`, `POSTGRES_11`, `POSTGRES_12`, `SQLSERVER_2017_STANDARD`,
 `SQLSERVER_2017_ENTERPRISE`, `SQLSERVER_2017_EXPRESS`, `SQLSERVER_2017_WEB`.
 [Database Version Policies](https://cloud.google.com/sql/docs/sqlserver/db-versions)
 includes an up-to-date reference of supported versions.