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/InsertSpec.hs

@@ -11,8 +11,8 @@ import Test.Hspec.Wai
 import Test.Hspec.Wai.JSON
 import Text.Heredoc
 
-import PostgREST.DbStructure.PgVersion (PgVersion, pgVersion112,
-                                        pgVersion130)
+import PostgREST.Config.PgVersion (PgVersion, pgVersion112,
+                                   pgVersion130)
 
 import Protolude  hiding (get)
 import SpecHelper