[WEB-1154] fix: delete attachment modal logic

[aaryan610]

Problem:

1. Trying to delete an attachment is not possible, the delete attachment modal only deletes the last attachment on the list.

Solution:

1. MobX store value to toggle the delete attachment modal was common for all the attachments, resulting in a conflict in the delete modals. Resolved the issue by using attachmentId to validate if the delete modal should be opened or not.

Plane issue: WEB-1154