Drop app/models/integrations from eager load paths

What does this MR do?

The models in this folder all use the new Integrations:: namespace, so they can be properly auto-loaded without having to add the path to eager_load_paths. This was only needed for the old project_services directory, which didn't use namespaces.

This fixes errors like this when referencing integration classes in the toplevel scope:

LoadError: Unable to autoload constant Github, expected ee/app/models/integrations/github.rb to define it

Instead we'll now get:

NameError: uninitialized constant Github

The previous workaround for the Jira integration to create an empty module in app/services/jira.rb isn't needed anymore, referencing a service like Jira::requests::Projects::ListService still works without raising an error.

Part of #201855 (closed), replaces the changes in !62437 (merged)

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.

Related to #201855 (closed)