feat: remove schema.typeName property from defaults

Remove `schema.typeName` from the default config in order to support elasticsearch v8.

Prior to version 6 it was recommended to be "_doc", in es7 it was optional (but if specified it was required to be "_doc"), in es8 it's not allowed at all.

See: https://www.elastic.co/guide/en/elasticsearch/reference/7.17/removal-of-types.html

BREAKING CHANGE: Pelias no longer supports elasticsearch v6

This change drops support for elasticsearch v6 in order to support v8.
The current recommended version remains v7.

Closes #142

config/defaults.json

@@ -70,8 +70,7 @@
     }
   },
   "schema": {
-    "indexName": "pelias",
-    "typeName": "_doc"
+    "indexName": "pelias"
   },
   "logger": {
     "level": "debug",

test/expected-deep.json

@@ -75,8 +75,7 @@
     }
   },
   "schema": {
-    "indexName": "pelias",
-    "typeName": "_doc"
+    "indexName": "pelias"
   },
   "logger": {
     "level": "debug",