Enable Merge Train checkbox based on Merge Pipelines checkbox

What does this MR do?

For issue #282520 (closed).

This sets the checkbox state for the Merge Options settings (under general project settings). The inital state is based on the results from querying ciCdSettings via GraphQL.

The behavior is as follows:

• If mergePipelinesEnabled is true, both "Enable merge results pipelines" and "Enable merge trains" checkboxes are enabled.
  • Unchecking "Enable merge results pipelines" will disable "Enable merge trains". If it was previously checked before,
• If mergePipelinesEnabled is false, "Enable merge trains" checkbox is unchecked and disabled.
  • Checking "Enable merge results pipelines" will enable "Enable merge trains". User can now check this option.
• If "Enable merge trains" checkbox is enabled, user can check or uncheck it without affecting anything else.

A follow-up issue has been created in #297354 to implement this behavior in Vue. For now, this will be implemented in vanilla JS since the form is in HAML.

Screenshots

Behavior	Screenshot
Both checkboxes enabled and checked
Merged Pipelines unchecked: unchecks and disables Merged Trains checkbox
Merged Trains unchecked: does not affect Merged Pipelines checkbox. Checking Merge Pipelines will enable the Merge Trains checkbox, but user has to check it manually.

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