Fix deprecation messages due to autoloading in initializers

What does this MR do?

This moves the loading of Zeitwerk earlier so that initializers are run with Zeitwerk already loaded (when we enable Zeitwerk).

This means the autoload paths would now be frozen when running initializers but that is fine in our case because we don't modify that in our initializers.

From https://github.com/rails/rails/pull/35745#issuecomment-476195706, Zeitwerk was intentionally loaded late so that initializers are more flexible and could mutate autoload paths, but like I said, we don't need that here and we actually need the autoloader because we reference a lot of application code there.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #197346 (closed)