Restrict the usage of value-description variables to root

What does this MR do?

Related to #30101 (closed)

The feature introduced by !43392 (merged) and !44120 (merged).

When introducing the pre-filled variables feature, we allow variables to be defined with value and description. However, we don't need to allow usage of this for job variables.

This MR just restricts the usage of this syntax to only root variables:

variables:
 TEST:
   value: "some value" # this would be the default value
   description: "This variable makes cakes delicious" # makes this variable appear on the Run Pipeline form

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