databases: Update Postgres versions in acceptance tests.

[andrewsomething]

We have acceptance tests failing:

    datasource_database_cluster_test.go:20: Step 1/2 error: Error running apply: exit status 1
        Error: Error creating database cluster: POST https://api.digitalocean.com/v2/databases: 422 (request "f1627cab-76fe-4d38-b86c-1f6460daed6a") invalid cluster engine version
          with digitalocean_database_cluster.foobar,
          on terraform_plugin_test.tf line 2, in resource "digitalocean_database_cluster" "foobar":
           2: resource "digitalocean_database_cluster" "foobar" {
--- FAIL: TestAccDataSourceDigitalOceanDatabaseCluster_Basic (0.91s)

Looks like version 11 has been EOLed for Postgres:

$ doctl databases options versions 
Engine     Versions
mysql      [8]
pg         [12,13,14,15]
redis      [7]
mongodb    [4.4,5.0,6.0]

[danaelhe]

nice catch 👍