Move advanced_search_multi_project_select FF to user actor

What does this MR do and why?

Related to #332631

The FF advanced_search_multi_project_select was originally setup to use a group actor, but we decided to not to limit it to group searches only so the actor will be changed to a user actor (#332631 (comment 674988297)). The FF will be rolled out using the percentage rollout for actors which should be a more stable way to roll it out for users consistently across searches.

This MR:

• update the milestone for the FF (which should have been previously updated from 14.0 but was missed
• change the FF to no longer use an actor
• refactor (and reduce data created) for spec

Screenshots or screen recordings

You can see results from both project_ids (6 - Flightjs / Flight and 44 - import test / gitlab-hq)

How to set up and validate locally

Note: You need to have Elasticsearch enabled in GDK, have Advanced Search enabled for search + indexing, and have created the index using rake commands or the Admin UI (rake gitlab:elastic:index)

1. Enable the feature in the rails console: Feature.enable(:advanced_search_multi_project_select, user) (user must be the one you log in with)
2. Navigate to the search page: http://localhost:3000/search
3. Take note of 2 project ids for any group (can be same or different group)
4. Manually add &project_ids=AA,BB to the URL where AA and BB are the project_ids from the previous step
5. perform some searches and make sure you are getting data back from both projects

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.