Rename custom hooks to server hooks

What does this MR do?

Rename custom hooks to server hooks. Per #196051 (closed).

• [-] Rename code and spec. I don't think there is any code related to this feature in GitLab itself.
• Rename the documentation

Not renamed:

• custom_hooks directory in gitlab-shell on per-project level. I believe it will cause more issues than benefits.

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

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