Drop existent index on database migration of 9.0

[github-actions]

{ key: { roomId: 1, userId: 1, messageId: 1 }, unique: true },

📝 Found in apps/meteor/ee/server/models/raw/ReadReceipts.ts#L14

Commit: f65cd9b

[AYUSHSAHU2004]

Hi, I’ve created a PR to address this issue.

PR: #40255

Please let me know if any changes are needed.