Refactor epic schema to use date as type

ee/spec/fixtures/api/schemas/public_api/v4/epic.json

@@ -24,15 +24,15 @@
         "type": "string"
       }
     },
-    "start_date": { "type": ["string", "null"] },
-    "start_date_fixed": { "type": ["string", "null"] },
-    "start_date_from_milestones": { "type": ["string", "null"] },
-    "start_date_is_fixed": { "type": ["boolean"] },
-    "end_date": { "type": ["string", "null"] },
-    "due_date": { "type": ["string", "null"] },
-    "due_date_fixed": { "type": ["string", "null"] },
-    "due_date_from_milestones": { "type": ["string", "null"] },
-    "due_date_is_fixed": { "type": ["boolean"] },
+    "start_date": { "type": ["date", "null"] },
+    "start_date_fixed": { "type": ["date", "null"] },
+    "start_date_from_milestones": { "type": ["date", "null"] },
+    "start_date_is_fixed": { "type": "boolean" },
+    "end_date": { "type": ["date", "null"] },
+    "due_date": { "type": ["date", "null"] },
+    "due_date_fixed": { "type": ["date", "null"] },
+    "due_date_from_milestones": { "type": ["date", "null"] },
+    "due_date_is_fixed": { "type": "boolean" },
     "created_at": { "type": ["string", "null"] },
     "updated_at": { "type": ["string", "null"] }
   },

ee/spec/fixtures/api/schemas/public_api/v4/epic_issue_link.json

@@ -18,15 +18,15 @@
         "group_id": { "type": "integer" },
         "description": { "type": ["string", "null"] },
         "author": { "type": ["object", "null"] },
-        "start_date": { "type": ["string", "null"] },
-        "start_date_fixed": { "type": ["string", "null"] },
-        "start_date_from_milestones": { "type": ["string", "null"] },
-        "start_date_is_fixed": { "type": ["boolean"] },
-        "end_date": { "type": ["string", "null"] },
-        "due_date": { "type": ["string", "null"] },
-        "due_date_fixed": { "type": ["string", "null"] },
-        "due_date_from_milestones": { "type": ["string", "null"] },
-        "due_date_is_fixed": { "type": ["boolean"] },
+        "start_date": { "type": ["date", "null"] },
+        "start_date_fixed": { "type": ["date", "null"] },
+        "start_date_from_milestones": { "type": ["date", "null"] },
+        "start_date_is_fixed": { "type": "boolean" },
+        "end_date": { "type": ["date", "null"] },
+        "due_date": { "type": ["date", "null"] },
+        "due_date_fixed": { "type": ["date", "null"] },
+        "due_date_from_milestones": { "type": ["date", "null"] },
+        "due_date_is_fixed": { "type": "boolean" },
         "created_at": { "type": ["string", "null"] },
         "updated_at": { "type": ["string", "null"] },
         "labels": {