Add variable for specifying database engine version (Pg v11 is not available anymore)

[annis-souames]

First, I would like to thank you for this project. I just want to raise an issue regarding the version of the DB cluster, currently in database.tf :

resource "digitalocean_database_cluster" "k3s" {
  name                 = "k3s-ext-datastore"
  engine               = var.database_engine == "postgres" ? "pg" : "mysql"
  version              = var.database_engine == "postgres" ? "11" : "8"
  size                 = var.database_size
  region               = var.region
  private_network_uuid = digitalocean_vpc.k3s_vpc.id
  node_count           = var.database_node_count
}

In this case if the database is PostgreSQL, the version used is 11, while as of now, the latest version in Digital Ocean is 16, with the oldest one being 12 so this is an issue and force users to go with MySQL v8. Would be nice to add a variable for this.