Move old integrations to Settings > General and introduce instance-level integrations

What does this MR do?

Context

In !25752 (merged), we moved some of the old "integrations" from Admin > Settings > Integrations to Admin > Settings > General. This was to make space for our new instance-level integrations (similar to project integrations - or as they were previously called - project services).

Changes

• Remove feature flag instance_level_integrations.
• Fix redirect path after saving of old integration forms.
• Remove stubs of instance_level_integrations set to false in specs and make sure all of them pass.

Screenshots

Section	Before	After
Admin > Settings > General
Admin > Settings > Integrations

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