API: Return activity for auth user’s projects in GET /events

What does this MR do?

This MR updates the GET /events endpoint to reflect the individual resources' activity feeds, instead of returning the activity generated by the current user.

To get activity for the authorized user, implementors now need to make a request to the GET /users/:id/events endpoint, with id set to the authorized user's id.

Are there points in the code the reviewer needs to double check?

I did not touch the existing tests, but everything still seems to pass.

Why was this MR needed?

GET /events is redundant given GET /users/{authorizedUser.id}/events can be used to achieve the same thing. With this change, GET /events can be used to power activity feeds for things of relevance to the current user (like the activity feed a user sees when logging into in Gitlab itself).

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug (see above things reviewer needs to double check)
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

This change is being discussed in #35887 (moved).