[RUN ALL RSPEC] [RUN AS-IF-FOSS] Switch to Zeitwerk autoloader

What does this MR do?

Switches from the classic autoloader to Zeitwerk. The classic autoloader is deprecated and will be removed in future version of Rails. Zeitwerk is also required for Sidekiq 6.

Previously, when we attempted to switch to Zeitwerk, we ran into code-reloading issues: #211875 (closed) specially when DB load balancing is enabled.

To verify that this is no longer the case:

1. Switch to this branch

2. Enable load balancing with something like this in config/database.yml:

   development:
     main:
       adapter: postgresql
       encoding: unicode
       database: gitlabhq_development
       pool: 5
       username: postgres
       password: password
       host: postgres
       load_balancing:
         hosts:
           - postgres
           - postgres

3. Start Puma

4. Browse the web UI

5. Run touch app/models/user.rb (Or any Ruby file)

6. Refresh the page

NOTE: We currently have a code-reloading issue in Sidekiq. When any Ruby file changes (like when you do touch above), the threads deadlock and when you check the process you'd eventually see something like sidekiq 5.2.9 gitlab [10 of 10 busy]. This MR does not solve the problem but it does not make it any worse. I'm planning to fix this separately.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.