Update manifest (#10228)

Author: JuliRossi

apps/slack/contentful-app-manifest.json

@@ -14,21 +14,17 @@
           "message": {
             "type": "string",
             "title": "Message Content",
-            "description": "The content of the message to be sent to Slack",
-            "minLength": 1,
-            "maxLength": 4000
+            "description": "The content of the message to be sent to Slack"
           },
           "channelId": {
             "type": "string",
             "title": "Slack Channel ID",
-            "description": "The Slack channel ID to send the message to",
-            "minLength": 1
+            "description": "The Slack channel ID to send the message to"
           },
           "workspaceId": {
             "type": "string",
             "title": "Slack Workspace ID",
-            "description": "The Slack workspace ID where the message should be sent",
-            "minLength": 1
+            "description": "The Slack workspace ID where the message should be sent"
           }
         },
         "required": [