docs: Update minimum Postgres supported version (#1216)

Per SPE-1347, we have to update the minimum version of Postgres that's
supported for Boundary. ~~On Nov 1, the minimum version changes from 13
to 16. This PR updates the enterprise supported versions topic.~~

Upon review, the team determined the minimum supported Postgres version
should be 14 currently. It will change to 15 on November 12. This PR
updates the current doc from 13 to 14. I updated
[SPE-1347](https://hashicorp.atlassian.net/browse/SPE-1347) to indicate
that it will have to be updated to 15 on November 12.


[SPE-1347]:
https://hashicorp.atlassian.net/browse/SPE-1347?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: Dan-Heath

content/boundary/v0.20.x/content/docs/enterprise/supported-versions.mdx

@@ -58,5 +58,5 @@ To view the Desktop version along with the corresponding CLI and control plane v
 
 ## PostgreSQL support policy
 
-Boundary Enterprise will only support PostgreSQL version 13 and above at launch.
+Boundary Enterprise supports PostgreSQL versions 14 and above.
 On an ongoing basis, Boundary will end support for a PostgreSQL version one (1) year prior to its EOL as documented on PostgreSQL’s versioning policy website.