refactor: move DbStructure.PgVersion to Config

Now that PgVersion is not part of DbStructure, Config is a more apt
module for it.

Also rename getDbStructure to queryDbStructure. AppState also had a
getDbStructure function for a record field.

test/Feature/QuerySpec.hs

@@ -8,9 +8,9 @@ import Test.Hspec          hiding (pendingWith)
 import Test.Hspec.Wai
 import Test.Hspec.Wai.JSON
 
-import PostgREST.DbStructure.PgVersion (PgVersion, pgVersion112,
-                                        pgVersion121, pgVersion96)
-import Protolude                       hiding (get)
+import PostgREST.Config.PgVersion (PgVersion, pgVersion112,
+                                   pgVersion121, pgVersion96)
+import Protolude                  hiding (get)
 import SpecHelper
 
 spec :: PgVersion -> SpecWith ((), Application)