[Bug] Retention Policy deletes entire Discussion if it started before prune date

Description:

Retention Policy deletes entire Discussion if it started before prune date even if that Discussion have a lot of recent messages that not affected by Retention Policy

@sampaiodiego please take a look at that issue.
@bseclier first informed me about that behavior here

Steps to reproduce:

1. Create Discussion e.g. at 1 of October and write a messages every day till now (until 16 of October included)
2. Set Retention Policy to prune all messages in all type of chats starting e.g. 10th of October
3. Discussion will be deleted entirely

Expected behavior:

All messages in Discussion since prune date (10 of October in example) should be remain in Discussion, and Discussion itself should not be deleted.

Actual behavior:

Discussion is gone with recent messages

Server Setup Information:

• Version of Rocket.Chat Server: 3.6.3
• Operating System: CentOS7
• Deployment Method: docker
• Number of Running Instances: 25
• DB Replicaset Oplog: Enabled
• NodeJS Version: v12.16.0
• MongoDB Version: 4.0.17

Client Setup Information

• Desktop App or Browser Version: Chrome 85
• Operating System: Windows 10

Additional context

Discussion should be fully deleted if all it's messages older then prune date (all made before 10 of October in example).
And it is true for all type of chats:

• Entire chat should be deleted of all it's messages are older then prune date
• Chat should remain it it have recent messages after prune date

Relevant logs:

server.js:204 Meteor ➔ method stream -> userId: null, arguments: ["notify-room","MvxB43F435T2FC2RuZD9QSjhLJiMQ5PRfS/deleteMessageBulk",{"0":{"rid":"MvxB43F435T2FC2RuZD9QSjhLJiMQ5PRfS","excludePinned":false,"ignoreDiscussion":false,"ts":{"$gte":"0001-01-01T00:00:00.000Z","$lte":"2020-08-01T09:00:00.838Z"},"users":[]}}]

Start with the retention-policy behavior described in the reproduction and trace the server.js:204 deleteMessageBulk log entry. Done means pruning removes only messages before the cutoff, while discussions and chats with newer messages remain; fully old chats may still be deleted.