[Maintenance] - Slack Event Notif Fixes

Description

Currently the event notification is hardcoded as Design Review for ${workPackageName} is being scheduled by ... but we should update this to work for all the new event types.

Additionally, the linkButtonText for confirming availability should only show up for events that are UNCONFIRMED. This is not applicable to confirmed events, or events where availability is not required

Acceptance Criteria

• Events that are not design reviews show up as "their event name for ${workPackageName} is being scheduled by" instead of hardcoded "Design Review"
• if an event doesn't require confirmation, or is already confirmed, the Slack notifs do not show 'Confirm Availability'

Proposed Solution

• In src/backend/src/utils/pop-up.utils.ts use the event type name for msg in sendEventPopUp
• In src/backend/src/utils/slack.utils.ts only show 'Confirm Availability' if the event status is unconfirmed and requires availability
• Test in #slackbot_land

Screenshots