fix(keyword): enum definition was relaxed in draft6

This PR _only_ updates drafts `2019-09` and `2020-12` because
the metaschemas weren't updated until draft2019-09.

Since draft5/6:
5.20. enum
 The value of this keyword MUST be an array. This array SHOULD have
 at least one element. Elements in the array SHOULD be unique.

Prior to draft5, the text reads:

5.5.1.1. Valid values
The value of this keyword MUST be an array. This array MUST have at least one element. Elements in the array MUST be unique.

Author: jeremyfiel

content/2019-09/validation/enum.markdown

@@ -1,7 +1,7 @@
 ---
 keyword: "enum"
 signature: "Array<Any>"
-value: This keyword must be set to a *non-empty* array of unique JSON values
+value: This keyword must be set to an array of unique JSON values
 summary: "Validation succeeds if the instance is equal to one of the elements in this keyword's array value."
 kind: [ "assertion" ]
 instance: [ "any" ]

content/2020-12/validation/enum.markdown

@@ -1,7 +1,7 @@
 ---
 keyword: "enum"
 signature: "Array<Any>"
-value: This keyword must be set to a *non-empty* array of unique JSON values
+value: This keyword must be set to an array of unique JSON values
 summary: "Validation succeeds if the instance is equal to one of the elements in this keyword's array value."
 kind: [ "assertion" ]
 instance: [ "any" ]
@@ -11,6 +11,8 @@ tests:
   - draft2020-12/enum.json
 index: -99998
 introduced_in: draft1
+changed_in:
+  - 2019-09
 related:
   - vocabulary: validation
     keyword: const