Add deployment events as supported by discord integration

What does this MR do and why?

Add deployment events as supported by discord integration.

There are not more changes needed to support deployment events. Adding it to the supported events and adjusting the specs is enough.

This is the second part of my plan to make the slack integration and the slack endpoint from discord replaceable with the discord integration (!125621 (comment 1462219166))

Screenshots or screen recordings

Old way with the slack integration	This MR without !133120 (merged)	This MR with !133120 (merged)

How to set up and validate locally

1. Setup a discord webhook

2. Setup the discord integration with that webhook and enable deployment events

3. Register a runner on GDK

4. Create the following .gitlab-ci.yml

   deploy:
       stage: deploy
       script:
           - echo hi
       environment: production

5. Trigger a pipeline and check the notifications in discord

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to Support Deployment Notifications in Discord, Ha... (#28278)