Add additional backup configurations to MongoDB Flex instance resource (#486)

* Extend resource and datasource

* Extend acc test

* Extend example

* Generate docs

* Fix linter

* Update stackit/internal/services/mongodbflex/instance/datasource.go

Co-authored-by: GokceGK <161626272+GokceGK@users.noreply.github.com>

---------

Co-authored-by: GokceGK <161626272+GokceGK@users.noreply.github.com>

examples/resources/stackit_mongodbflex_instance/resource.tf

@@ -11,9 +11,10 @@ resource "stackit_mongodbflex_instance" "example" {
     class = "class"
     size  = 10
   }
-  version = "5.0"
+  version = "7.0"
   options = {
-    type = "Single"
+    type                    = "Single"
+    snapshot_retention_days = 3
   }
   backup_schedule = "0 0 * * *"
 }